When I tried to load my Roundcube today, found that it failed to load the inbox and instead had thrown the following error:
Server Error! (Ok)
Then, I tried searching the cpanel logs or the roundcube error log but found nothing. Then, I checked the Dovecot log located here:
/var/log/maillog
I found the following:
May 7 13:57:49 network2 dovecot: imap([email protected])<26343><cQG+qxX7MvdneMrv>: Error: Mailbox INBOX: mmap(size=351817308) failed with file /home/mellow/mail/mellowhost.com/shawon/dovecot.index.cache: Cannot allocate memory
This is happening because Dovecot caches the mail index in a file, once it tries to cache a lot of emails, it fails with a memory error. In those cases, you may remove the cache file and let Dovecot generate a new cache based on the latest mails. You may simply rm the file and see Roundcube is loading again:
rm -f /home/mellow/mail/mellowhost.com/shawon/dovecot.index.cache