Sounds like corruption someplace
You mentioned 30k items on the laptop. I take it that is a POP3 setup?
Missing mail is usually due to the mailbox file becoming too large, index files getting out-of-sync, or outright filesystem corruption. The first two are on the application, the last one largely on the OS and underlying filesystem.
Rejected passwords are usually due to issues on the server side. Comcast does it to us every few months. But I'm not familiar enough with Apple Mail to rule out an interaction with other issues.
You can check for filesystem trouble with the Disk Utility or fsck from single user mode.
https://www.computerworld.com/article/2476217/mac-os-x-how-to-fix-apple-mail-by-rebuilding-and-reindexing-the-mailbox.html
The link is an old-ish article on repairing mail file indexes. YMMV as it sounds like you may not even be able to get to the menu.
Missing mail is usually due to the mailbox file becoming too large, index files getting out-of-sync, or outright filesystem corruption. The first two are on the application, the last one largely on the OS and underlying filesystem.
Rejected passwords are usually due to issues on the server side. Comcast does it to us every few months. But I'm not familiar enough with Apple Mail to rule out an interaction with other issues.
You can check for filesystem trouble with the Disk Utility or fsck from single user mode.
https://www.computerworld.com/article/2476217/mac-os-x-how-to-fix-apple-mail-by-rebuilding-and-reindexing-the-mailbox.html
The link is an old-ish article on repairing mail file indexes. YMMV as it sounds like you may not even be able to get to the menu.