Thunderbird is pretty nice as a platform. Simply like with Firefox, People dont get that. So Addons need to be preinstalled.
- DKIM verifier is important and unintrusive.
- Ublock Origin. I currently use UBO light (downloaded the .xpi file and installed manually) mostly just to autoblock malware
- something with clamav. This needs to be integrated in some way, antivirus and mails are a pretty common combo but ClamAV is a good tool thats not user friendly. Often it is used on the server side, but you cant really rely on that
Also, having sync between phone and desktop is important. On Android I simply have my contact storage, this is not there on Linux. I have .vcf files, but thats subpar. Having sync only the contacts with mails from phone to Desktop would be great!
The lack of a unified and protected location for contact storage could be easily circumvented by storing them simply in Thunderbird, syncing them through Thunderbird etc, as its already the best Calendar and contact manager imho on Linux.