Local email delivery to an account fails with "LMTP connection closed after end of data" error

Symptoms

You send a message to a local email account. The message fails to deliver, and this error appears in the /var/log/exim_mainlog file:

R=virtual_user T=dovecot_virtual_delivery defer (-1): LMTP connection
closed after end of data

Description

This issue can occur when there is mdbox corruption on the email account. You can also see evidence of corruption like this in the /var/log/maillog file:

Dec 24 04:21:36 hotel dovecot: pop3(test@domain.com)<4511><7Ru4JC63tpvRVack>: Error:
Corrupted dbox file /home/domain/mail/domain.com/info/storage/m.742 (around offset=0):
EOF while reading file header

Workaround

If the corrupted file is of zero size, you can remove or move it aside, then run this command to resynchronize the account's Inbox:

doveadm -Dv force-resync -u test@domain.com INBOX

Otherwise, you can try converting the account to maildir, then back to mdbox on the Mailbox Conversion page in WHM. If doing that does not resolve the issue, you will need to restore the account from a backup that does not have the corrupted file.