When comparing gnus vs Mozilla Thunderbird, the Slant community recommends Mozilla Thunderbird for most people. In the question“What are the best native e-mail clients for Linux?” Mozilla Thunderbird is ranked 1st while gnus is ranked 12th.
Ranked in these QuestionsQuestion Ranking
Pro It supports IMAP and POP3
… but you can use DavMail to connect to Exchange server via EWS.
Pro Everything is customizable
Yep, really. From Outlook-mode, where Outlook-typical TOFU mails are “repaired”, up to a high-sophisticated scoring and SPAM filtering, footer lines with quotes, rules depending on the recipient, MIME formatting, boxquotes, etc.
Pro Free (as in freedom and beer)
Pro Lightning Calendar and Address Book integration
Pro Available on Linux, OS X and Windows
Pro Good filtering system
Mozilla Thunderbird offers a flexible filtering system with the ability to set flags and read/unread, as well as sort/assign to new mail directories. There are also numerous plugins available to assist in the filtering.
Pro Very reliable
Version after version, Mozilla Thunderbird works as expected.
Pro Consistent and involving a moderate learning curve
The app design of Mozilla Thunderbird has not changed significantly since its beginning, making its learning curve almost non-existent.
Pro Large assortment of plugins available
Mozilla Thunderbird offers a huge amount of extensions to expand the usability and options of the client.
Pro Tabs for navigation
Pro Send large files easy
Install pCloud plugin for Thunderbird and your files (upto 20 GB free storage included) will be received as download links
Con Difficult documentation
Originally a newsgroup reader, so its documentation uses Usenet terminology for everything -- which is confusing.
Con Terrible integration with macOS
Simple things - drag/dropping pictures doesn't work, for instance.
Con Development slowed to only focus on stability and security
Mozilla Thunderbird has slowed down its development of features to only focus resources on security and stability.
Con Subject lines can (temporarily) disappear from the list
This is sporadic behavior.
Con Serious bugs
Clicking on a subject line can bring up the wrong email.
Con Outdated interface
Thunderbird's interface is very outdated and unpleasant. It feels more like a Windows XP application than like a modern MacOS one.