Filed by FirstMark Horizon Acquisition Corp.

Pursuant to Rule 425 under the Securities Act of 1933,

as amended, and deemed filed pursuant to Rule 14a-12

under the Securities Exchange Act of 1934, as amended

Subject Company: FirstMark Horizon Acquisition Corp.

Commission File No.: 001- 39585

Email to Starry Subscribers

SUBJECT:	Starry Begins Process to Become a Public Company
TO:	Starry Subscribers
FROM:	Chet Kanojia
DATE:	October 7, 2021

To our #HappyInterneting Family,

When we set out to build this business, we had a belief, backed by data and our own personal experiences, that the broadband industry in the US was broken. It was non-competitive, it was expensive, and the customer experience left a lot to be desired. Our team had a deep conviction that we could build breakthrough technology that would transform the industry. Our mission was to bring more competition and choice to consumers, but most importantly, our goal was to deliver an internet service experience that was delightful -- from the time you signed up, through your lifecycle as a subscriber.

For the last six years, we’ve been on an intense journey to do exactly that. Focused on our mission and our customers, we’ve brought Starry to subscribers across Boston, Los Angeles, New York, DC, Denver and soon, in Columbus. Our customer-first focus on our service and the products we build continues to guide us; our subscribers motivate and inspire us daily. That’s why I’m proud to share with you today that Starry has taken its first steps to becoming a public company.

The last 18 months have reaffirmed our belief that broadband internet is essential to every community and every household in the United States. Starry’s technology and service model is well-positioned to bring an innovative and affordable option to millions of American households. Our team has made remarkable progress, building a network that covers more than 4.7 million homes, including yours.

Growing at this rapid pace, requires significant investment in our business, so today, we are excited to announce a big step in Starry’s growth: we have begun the process of becoming a publicly traded company by agreeing to merge with FirstMark Horizon Acquisition Corp., a special purpose acquisition company sponsored by an affiliate of FirstMark Capital. This merger will provide us with access to the capital required to help us to bring Starry to even more cities and homes across the US.

Importantly, this step does not mean any significant changes in your service. We will still be the same Starry you know and love - laser-focused on bringing choice, competition, and affordability to the marketplace and on growing the reach of our Starry Connect program. This merger means we’ll be better equipped to grow and deliver for more people in new markets across the country.

As one of our earliest investors, FirstMark Capital has been on this journey with Starry from the beginning. They have a great track record of investing in and supporting companies from their earliest days, through to becoming public companies and beyond. FirstMark Horizon Acquisition Corp., knows our business and our technology, but importantly, they believe in our mission and customer-first focus. That’s why we feel confident in taking this next important step with FirstMark Horizon Acquisition Corp., as our partner.

Our customers have always been at the heart of our business and we’re grateful that you continue to be on this journey with us. This next phase of our company is an important one and we’re excited for where it will take us.

Stay safe and thank you for being a part of Starry’s #HappyInterneting family.

Yours truly,

Chet

Additional Information about the Business Combination and Where to Find It

In connection with the proposed business combination, Starry Holdings, Inc. (“Starry Holdings”), a newly formed subsidiary of Starry, Inc. (“Starry”), will file a registration statement on Form S-4 (the “Form S-4”) with the Securities and Exchange Commission (the “SEC”). The Form S-4 will include a proxy statement of FirstMark Horizon Acquisition Corp. (“FirstMark”) and a prospectus of Starry Holdings, referred to as a proxy statement/prospectus. The proxy statement/prospectus will be sent to all FirstMark stockholders. Additionally, Starry Holdings and FirstMark will file other relevant materials with the SEC in connection with the proposed business combination. Copies of the Form S-4, the proxy statement/prospectus and all other relevant materials filed or that will be filed with the SEC may be obtained free of charge at the SEC’s website at www.sec.gov. Before making any voting or investment decision, investors and security holders of FirstMark are urged to read the Form S-4, the proxy statement/prospectus and all other relevant materials filed or that will be filed with the SEC in connection with the proposed business combination because they will contain important information about the proposed business combination and the parties to the proposed business combination.

Participants in Solicitation

FirstMark, Starry Holdings and Starry and their respective directors and executive officers, under SEC rules, may be deemed to be participants in the solicitation of proxies of FirstMark’s stockholders in connection with the proposed business combination. Investors and security holders may obtain more detailed information regarding the names and interests in the proposed business combination of FirstMark’s directors and officers in FirstMark’s filings with the SEC, including FirstMark’s registration statement on Form S-1, which was originally filed with the SEC on September 18, 2020. To the extent that holdings of FirstMark’s securities have changed from the amounts reported in FirstMark’s registration statement on Form S-1, such changes have been or will be reflected on Statements of Change in Ownership on Form 4 filed with the SEC. Information regarding the persons who may, under SEC rules, be deemed participants in the solicitation of proxies to FirstMark’s stockholders in connection with the business combination will be included in the proxy statement/prospectus relating to the proposed business combination when it becomes available. You may obtain free copies of these documents as described in the preceding paragraph.

No Offer or Solicitation

This communication shall not constitute a proxy statement or solicitation of a proxy, consent or authorization with respect to any securities or in respect of the proposed business combination. This communication shall also not constitute an offer to sell or a solicitation of an offer to buy any securities of FirstMark, Starry Holdings or Starry, nor shall there be any sale of securities in any state or jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state or jurisdiction. No offering of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act of 1933, as amended.

2

Forward-Looking Statements

This communication includes “forward-looking statements” within the meaning of the “safe harbor” provisions of the Private Securities Litigation Reform Act of 1995 with respect to the proposed business combination between FirstMark and Starry. Words such as “expect,” “estimate,” “project,” “budget,” “forecast,” “anticipate,” “intend,” “plan,” “may,” “will,” “could,” “should,” “believe,” “predict,” “potential,” “continue,” “strategy,” “future,” “opportunity,” “would,” “seem,” “seek,” “outlook” and similar expressions are intended to identify such forward-looking statements. Forward-looking statements are predictions, projections and other statements about future events that are based on current expectations and assumptions and, as a result, are subject to risks and uncertainties that could cause the actual results to differ materially from the expected results. These statements are based on various assumptions, whether or not identified in this communication. These forward-looking statements are provided for illustrative purposes only and are not intended to serve as, and must not be relied on by an investor as, a guarantee, an assurance, a prediction or a definitive statement of fact or probability. Actual events and circumstances are difficult or impossible to predict and will differ from assumptions. These forward-looking statements include, without limitation, Starry’s and FirstMark’s expectations with respect to anticipated financial impacts of the proposed business combination, the satisfaction of closing conditions to the proposed business combination, and the timing of the completion of the proposed business combination. You should carefully consider the risks and uncertainties described in the “Risk Factors” section of FirstMark’s registration statement on Form S-1 (File No. 333-248916), its Annual Report on Form 10-K, as amended from time to time, for the fiscal year ended December 31, 2020 and its subsequent Quarterly Reports on Form 10-Q. In addition, there will be risks and uncertainties described in the Form S-4 and other documents filed by FirstMark or Starry Holdings from time to time with the SEC. These filings would identify and address other important risks and uncertainties that could cause actual events and results to differ materially from those contained in the forward-looking statements. Most of these factors are outside Starry’s, Starry Holdings’ and FirstMark’s control and are difficult to predict. Many factors could cause actual future events to differ from the forward-looking statements in this communication, including but not limited to: (1) the outcome of any legal proceedings that may be instituted against FirstMark, Starry or Starry Holdings following the announcement of the proposed business combination; (2) the inability to complete the proposed business combination, including due to the inability to concurrently close the business combination and related transactions, including the private placements of common stock and convertible notes or due to failure to obtain approval of the stockholders of FirstMark; (3) the risk that the proposed business combination may not be completed by FirstMark’s business combination deadline and the potential failure to obtain an extension of the business combination deadline if sought by FirstMark; (4) the failure to satisfy the conditions to the consummation of the proposed business combination, including the approval by the stockholders of FirstMark, the satisfaction of the minimum trust account amount following any redemptions by FirstMark’s public stockholders and the receipt of certain governmental and regulatory approvals; (5) delays in obtaining, adverse conditions contained in, or the inability to obtain necessary regulatory approvals or complete regulatory reviews required to complete the proposed business combination; (6) the occurrence of any event, change or other circumstance that could give rise to the termination of the merger agreement; (7) volatility in the price of FirstMark’s, Starry’s or Starry Holdings’ securities; (8) the risk that the proposed business combination disrupts current plans and operations as a result of the announcement and consummation of the business combination; (9) the inability to recognize the anticipated benefits of the proposed business combination, which may be affected by, among other things, competition, the ability of the combined company to grow and manage growth profitably, maintain relationships with customers and suppliers and retain key employees; (10) costs related to the proposed business combination; (11) changes in the applicable laws or regulations; (12) the possibility that the combined company may be adversely affected by other economic, business, and/or competitive factors; (13) the risk of downturns and a changing regulatory landscape in the highly competitive industry in which Starry operates; (14) the impact of the global COVID-19 pandemic; (15) Starry’s ability to obtain or maintain rights to use licensed spectrum in any market in which Starry operates and potential declines in the value of Starry’s FCC licenses; (16) the potential inability of Starry to raise additional capital needed to pursue its business objectives or to achieve efficiencies regarding other costs; (17) the enforceability of Starry’s intellectual property, including its patents, and the potential infringement on the intellectual property rights of others, cyber security risks or potential breaches of data security; and (18) other risks and uncertainties described in FirstMark’s registration statement on Form S-1 and Annual Report on Form 10-K, as amended from time to time, for the fiscal year ended December 31, 2020 and its subsequent Quarterly Reports on Form 10-Q. These risks and uncertainties may be amplified by the COVID-19 pandemic, which has caused significant economic uncertainty. Starry, Starry Holdings and FirstMark caution that the foregoing list of factors is not exclusive or exhaustive and not to place undue reliance upon any forward-looking statements, including projections, which speak only as of the date made. None of Starry, Starry Holdings or FirstMark gives any assurance that Starry, Starry Holdings or FirstMark will achieve its expectations. None of Starry, Starry Holdings or FirstMark undertakes or accepts any obligation to publicly provide revisions or updates to any forward-looking statements, whether as a result of new information, future developments or otherwise, or should circumstances change, except as otherwise required by securities and other applicable laws.

3