Losing emails

[H <ag...@me...>]

I have a problem tangentially related to mbsync and backing up mailboxes that I would appreciate any thoughts on.

The email provider is Ionos, formerly 1&1, and the IMAP mailboxes are accessed by one computer and multiple phones/tablet. The computer uses Thunderbird (CentOS) albeit an older version, 52.9.1, due to problems with task lists in later versions, and the phones/tablet use K-9 mail (Android).

I sometimes find that blocks (weeks/months) of emails have unexpectedly disappeared from Thunderbird/K-9 mail and can no longer be found even when accessing the mailbox using the web interface (which never really use.)

I am stumped why this would happen, I only delete one e-mail at at time using any of the e-mail programs and I never compress the mailboxes in Thunderbird. Although I use Mbsync to back up my mailboxes which helps me later retrieve lost e-mails, I need to understand why the above happens. An unreliable mailstore like this is a disaster...

Does anyone have any thoughts?