Hi, I'm leaving this here for others who may see the same issue and are equally perplexed as I was:
Dovecot version 2.3.16 w/ sdbox mailbox format.
Yesterday, I noticed that I did not see any new mail in Thunderbird. But for whatever reason, new mail did show up on my phone with K9 mail. So, new mails were retained.
In the Dovecot logs, I was seeing multiple errors: Corrupted dbox file read() failed for file No buffer space available EOF while reading file header Mailbox INBOX: sdbox: Rebuilding failed Mailbox INBOX: Deleting corrupted cache: reset_id mismatch even after locking
The "Corrupted dbox file" and "EOF while reading file header" error was caused by a message file in the INBOX mailbox that was 6681 bytes in size and had all NUL characters. File date of that file was more than two years ago. The "No buffer space available" error was caused by a file that also had a lot of NUL, but also a few other characters in it. Nothing resembling regular message files with headers and body. In total it was a handful of files causing issues. I deleted them all with:
doveadm expunge -u {user} MAILBOX INBOX UID {filename without the leading 'u.'}
Afterwards Thunderbird showed new mails again. I did not need to restart it. No more errors in the Dovecot logs.
Not sure what had caused this. Maybe somebody has seen this before.
Simon