MailMate crashes while starting/syncing

I am getting a crash with version 6016 of MM. I have three accounts and on start up MM crashes. I followed the advice someone posted in 2019 to:

1. Turn off wifi
   
2. Open MM
   
3. Take every mailbox offline
   
4. Shut down MM
   
5. Turn wifi on
   
6. Open MM
   
7. Bring each mailbox back online one by one

At the very least if the app crashes you know which mailbox is causing this. You can just leave the mailbox account offline to resolve it later. This at least gets MM up and running on some of your email accounts.

BTW, I also use GPG but the crash wasn't caused by an encrypted email I sent or received.

Same crash (?), new info:

I am also using Fastmail; I am not using GPG.

Over the weekend I used MailMate's "export to mbox" function, so that I could drop the resulting copy of mbox into GoodByEmail, and discover some junk I could delete from my mail repository.

(For clarity: I believe that the export function created a copy of my data (in mbox format) that MailMate then ignores, and that GoodByEmail can then use for analysis purposes. In other words, altering an exported mbox should have no effect on MailMate's functioning.)

That all worked, and so I then did a bunch of deletion from within MailMate, by GoodByEmail's analysis. All of this seemed routine. I ended with a smaller IMAP blob on fastmail, as I expected.

Then, when I next launched MailMate, the UI popped up for about three seconds, then threw a SegFault:

Process: MailMate [10004] Path: /Applications/MailMate.app/Contents/MacOS/MailMate Identifier: com.freron.MailMate Version: 1.14 (5937) Code Type: ARM-64 (Native) Parent Process: launchd [1] User ID: 501

Date/Time: 2024-01-28 12:41:07.1836 -0800 OS Version: macOS 14.2.1 (23C71) Report Version: 12

...

Crashed Thread: 0 Dispatch queue: com.apple.main-thread

Exception Type: EXC_BAD_ACCESS (SIGSEGV) Exception Codes:
KERN_INVALID_ADDRESS at 0x000000029733806e Exception Codes:
0x0000000000000001, 0x000000029733806e

Termination Reason: Namespace SIGNAL, Code 11 Segmentation fault: 11 Terminating Process: exc handler [10004]

Reinstalled MailMate using the latest recommended version. Also no go.

Trying to figure out where all IMAP info is cached in the hopes that I can delete the local IMAP cache without deleting e.g. my signature and other preferences. I figure there's a decent chance that I'd then be able to launch the app, re-sync to the server (over however many minutes or hours that would take) and then be back on my feet.

Following Klay's advice above, confirmed that Gmail is not causing the crash and Fastmail is.

A-hah! Help > Rebuild Database got me back up and running.

In particular, printf "OldDatabase" > ~/Library/Application\ Support/MailMate/.rebuild.tmp was a very helpful find.