Look, I haven't chanced the conf, but I've had these problems since
19.10:

May 11 11:11:32 mail dovecot: imap(mac)<1488494><mmGrRmOllcjMehAG>: Error: 
Mailbox Trash: copy: i_stream_read(/var/spool/mail/mac) failed: Cached message 
size larger than expected (5548 > 5547, box=INBOX, UID=27882)
May 11 11:12:05 mail dovecot: imap(mac)<1488582><XkqwSGOlzcjMehAG>: Error: 
Corrupted record in index cache file 
/var/dovecot-index/mac/.imap/INBOX/dovecot.index.cache: UID 20762: Broken 
physical size in mailbox INBOX: read(/var/spool/mail/mac) failed: Cached 
message size larger than expected (5174 > 5173, box=INBOX, UID=20762)
May 11 11:12:05 mail dovecot: imap(mac)<1488582><XkqwSGOlzcjMehAG>: Error: 
Mailbox Trash: copy: i_stream_read(/var/spool/mail/mac) failed: Cached message 
size larger than expected (5174 > 5173, box=INBOX, UID=20762)
May 11 11:27:11 mail dovecot: imap(ariex)<1491563><3ZvEfmOlPBeu2Bgu>: Error: 
Mailbox Trash: lseek() failed with mbox: Illegal seek
May 11 11:42:18 mail dovecot: imap(ariex)<1491563><3ZvEfmOlPBeu2Bgu>: Error: 
Mailbox Trash: lseek() failed with mbox: Illegal seek
May 11 11:57:23 mail dovecot: imap(ariex)<1491563><3ZvEfmOlPBeu2Bgu>: Error: 
Mailbox Trash: lseek() failed with mbox: Illegal seek
May 11 12:05:42 mail dovecot: imap(wstewart)<1499301><8/6ICGSlwswYEDrL>: Error: 
Next message unexpectedly corrupted in mbox file /var/spool/mail/wstewart at 
492309796

      Surely something isn't right.

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On Mon, 11 May 2020, Christian Ehrhardt  wrote:

> Date: Mon, 11 May 2020 11:38:01 -0000
> From: Christian Ehrhardt  <email address hidden>
> To: <email address hidden>
> Subject: [Bug 1876537] Re: dovecot eating mail indexes /var/dovecot-index/user
> 
> I'm wondering about the temper here, you got three responses in four days.
> Where is that a lack of attention or "being ignored".
>
> All I see is:
> - Paride was asking for more info than "Something is happening to the dovecot-index file" which seems fine to me.
> - Lucas later marked it invalid after your debugging showed that it doesn't appear as a bug in the dovecot code or its packaging.
> - Rafael tried to explain options if you want a higher support level. You might not want that, but there is nothing bad in him telling you I'd think.
>
> Finally you said " I will just compile it myself" - does that imply there is indeed a fix that makes it handle corrupted inboxes more gracefully? If so could you point to a commit or upstream discussion.
> I'm asking as we are most likely going to bump 2.3.7.2 to 2.3.10 this cycle and if your fix is upstream we should get it by that for Ubuntu 20.10 and later. But furthermore the questions will then become if there is an isolated fix that is SRU-backportable - so if you have identified one please let us know.
>
> -- 
> You received this bug notification because you are subscribed to the bug
> report.
> https://bugs.launchpad.net/bugs/1876537
>
> Title:
>  dovecot eating mail indexes /var/dovecot-index/user
>
> Status in dovecot package in Ubuntu:
>  Invalid
>
> Bug description:
>       I will get reports of users being able to connect, authenticate, but then not have any mail
>  available when there is new mail in their INBOX.  Something is happening to the dovecot-index file
>  for that user because deleting it restores their ability to get mail.
>
>       I have never had this happen before in six years of using dovecot, it is only since upgrade
>  to 20.04 that this started happening.  I made no local conf changes.
>
>  ProblemType: Bug
>  DistroRelease: Ubuntu 20.04
>  Package: dovecot-core 1:2.3.7.2-1ubuntu3 [modified: lib/systemd/system/dovecot.service]
>  Uname: Linux 5.6.0 x86_64
>  ApportVersion: 2.20.11-0ubuntu27
>  Architecture: amd64
>  CasperMD5CheckResult: skip
>  Date: Sat May  2 17:23:01 2020
>  ProcEnviron:
>   TERM=xterm-color
>   PATH=(custom, no user)
>   XDG_RUNTIME_DIR=<set>
>   LANG=en_US.UTF-8
>   SHELL=/bin/bash
>  SourcePackage: dovecot
>  UpgradeStatus: No upgrade log present (probably fresh install)
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/ubuntu/+source/dovecot/+bug/1876537/+subscriptions
>