13D Filing: True Wind Capital, L.p. and Zix Corp (NASDAQ:ZIXI)

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Item 1.

Security and Issuer

This Schedule 13D (the Schedule 13D) relates to the shares of common stock, par value $0.01 per share (the Common Stock), of Zix
Corporation, a Texas corporation (Zix or the Issuer). The principal executive offices of the Issuer are located at 2711 North Haskell Avenue, Suite 2200, LB 36, Dallas, Texas 75204-2960.

Item 2.

Identity and Background

(a) and (f) This Schedule 13D is being filed jointly on behalf of the following persons (collectively, the Reporting Persons):

1. Zephyr Holdco, LLC, a Delaware limited liability company (Zephyr Holdco),

2. True Wind Capital, L.P., a Delaware limited partnership (True Wind Capital and, together with Zephyr Holdco, True
Wind),

3. True Wind Capital GP, LLC, a Delaware limited liability company (True Wind Capital GP),

4. James H. Greene, Jr., a United States citizen, and

5. Adam H. Clammer, a United States citizen.

The Reporting Persons have entered into an agreement of joint filing, a copy of which is attached hereto as Exhibit A.

(b) and (c) The manager of Zephyr Holdco is True Wind Capital. The general partner of True Wind Capital is True Wind Capital GP. Mr. Greene and
Mr. Clammer are the managing members of True Wind Capital GP.

The principal business of Zephyr Holdco is to invest in securities of the Issuer. The
principal business of True Wind Capital is to serve as the manager of Zephyr Holdco and to manage investments through other partnerships and limited liability companies. The principal business of True Wind Capital GP is to serve as the general
partner of True Wind Capital and to manage investments through other partnerships and limited liability companies. Each of Mr. Greene and Clammer are the managing members of True Wind Capital GP.

The principal office of each of the Reporting Persons is located at c/o True Wind Capital, Four Embarcadero Center, Suite 2350, San Francisco, CA 94111.

(d) and (e) None of the Reporting Persons has, during the past five years, been convicted of any criminal proceeding (excluding traffic violations or
similar misdemeanors), nor been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or
prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

Item 3.

Source and Amount of Funds or Other Consideration

The information set forth in or incorporated by reference in Item 6 of this Schedule 13D is hereby incorporated by reference in its entirety into this Item 3.

On January 14, 2019, the Issuer entered into an Investment Agreement (the Investment Agreement) with True Wind Capital. Pursuant to the
Investment Agreement, on February 20, 2019, (i) Zephyr Holdco purchased from the Issuer 64,914 the Issuers Series A Convertible Preferred Stock (Series A Preferred Stock) and 35,086 shares of the Issuers Series B
Convertible Preferred Stock (Series B Preferred Stock and, together with the Series A Preferred Stock, the Preferred Stock) for an aggregate purchase price of $100 million.

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