It was originally released in 1990[1] on NetWare networks[3] with MS-DOS and later Apple Macintosh clients, before being ported to Windows which is now the only platform actively supported.

Pegasus Mail pioneered many features now taken for granted with other email clients, such as filtering and simultaneous access to multiple POP3 and IMAP4 accounts.

In January 2007 it was announced that distribution and development of Pegasus Mail had ceased due to inadequate financial support from the sale of the manuals.

[10][11] Harris stated:[12] We regret this decision, but ongoing difficulties with funding have forced it upon usLater in the month, due to an "avalanche" of support from the user community, it was announced that development would resume.

Since Pegasus Mail does not make changes to the Windows registry or the system directory, it is suitable as a portable application for USB drives.

Malicious HTML tends to be highly dependent on the exact target application and OS, therefore by avoiding both the ubiquitous HTML renderer supplied with Windows and not allowing automation commands such as ActiveX and JavaScript to execute from within an email in its inbuilt renderer, Pegasus reduces substantially the risk of infection from viewing email.

Release 4.41 also has, amongst other features, an improved HTML rendering engine, better support for special character encoding (especially with UTF-8), phishing protection, and a full-fledged Bayesian spam filter.

Pegasus Mail's takes an "old-fashioned" approach with advantages for knowledgeable users with complex email usage patterns, or who need special features.

Some examples include:[7] The early versions of Pegasus were installed on MS-DOS or Mac workstations on a NetWare network (and their Message Handling System (MHS) mail system), and supported only mail between network users; for external (Internet), Mercury for Netware was required.

)[21] The Mac version (2.21 from 1997) can be found on some FTP servers that in the past offered an official Pegasus mirror service.

[27] On 2 November 2010 Harris posted a message regarding the progress of development on the next release of Pegasus Mail, PMail Version 5.0.

I'm personally very excited about the new contact manager - I believe it could have an enormous impact on the way many people work in a world of e-mail overload" On 23 February 2011 Pegasus Mail 4.61 was released.

On 25 December 2019 Harris said that, while there has been a delay due to health issues, he "can only promise you that there is progress, and that [he is] totally committed to getting these new versions released" and he is working, among others, on support for OAuth2 and OpenSSL v 1.1.1.