Bugs, bugs, bugs, ...
Let's start with the bad news. At time of writing, 20th August 2021, Bugzilla shows 14.312 open "tickets" in Thunderbird (components Thunderbird: 7828, Mailnews Core: 4257, Calendar: 1996, Chat Core: 231, Bugzilla can't retrieve more than 10.000 bugs, you need to query separately), not counting the bugs in the Mozilla platform code that affect Thunderbird (like in the Editor, Spellchecker, Serialiser, etc.). Not all "tickets" are bugs, of course, but a fair portion is. Chances are that you are experiencing one of those.
We are here to help
So what do to? In general, we're here to help, but as you will understand, our small team cannot fix all 10.000+ bugs given that some of them are two decades old and the Thunderbird team which has many more resources than we do haven't addressed those bugs. As a first step, we suggest to install the current ESR 91 version of Thunderbird in parallel and see whether the issue exists there. If so, it's a good idea to check whether a bug has been reported already at Bugzilla. If the issue only exists in Betterbird, please contact us immediately and we will endeavour to get a fix to you as soon as possible. If the issue exists in Thunderbird, regardless of whether it has been reported at Bugzilla or not, you can contact us and we can check how hard it would be to fix the issue. Generally the idea of Betterbird is that our team tries to provide users with a better experience and that involves fixing long-standing and annoying bugs that somehow the Thunderbird team is not addressing.
Is Betterbird a virus?
We have seen cases where Windows Security makes you believe that Betterbird contains malicious code, we got a report for "Trojan:Script/Wacatac.B!ml" with the wording "This program is dangerous and executes commands from an attacker". Scary stuff. We've since started self-signing all executables and will use a proper certificate once the financial situation allows it. We've also excluded the Betterbird program directory and process from further scans.
Betterbird doesn't contain any more malicious code than the Thunderbird or Mozilla codebase, but we admit that we haven't inspected the millions of lines of upstream code. The binaries we distribute are built from open source code and you can build the package yourself using the instruction on the Github repository. You can also submit alleged malicious code to an online virus scanner like VirusTotal or Jotti.
One word of caution: E-Mail in general is dangerous. Like all written communication, it doesn't convey any facial expression, tone of voice and irony is frequently misunderstood. So use e-mail wisely and don't pick a fight! Also, e-mail is dangerous when sending the wrong stuff to the wrong people. Please check twice to whom you're sending the message before clicking "Send". We speak from experience :-( - If you're transmitting confidential information, you should consider setting up OpenPGP or using p≡p
Lastly and humorously: Some consider the Windows operating system to be a boot sector virus. We hope Betterbird will spread like a virus, and we need you to make the project a success.