LMTP error after RCPT TO::
451 4.3.0 Temporary internal error: retry timeout exceeded

dovecot: lmtp(5768): Error: fchown(/home/user/mail/domain.com/info/maildirsizesrv.domain.com.5768.87dedd4b02379106, group=12(mail)) failed: Operation not permitted (egid=507(user), group based on /home/user/mail/domain.com/info – see http://wiki2.dovecot.org/Errors/ChgrpNoPerm)
lmtp(6918): Error: safe_mkstemp(/home/user/mail/domain.tld/info/maildirsize) failed: Operation not permitted

Its cPanel issue, so you cha try update [email protected] mailbox quota to new: Allocated Storage Space