El error:
dovecot imap(username)<#####><###########+##############/+#/#>: Error: Failed to read mbox file /var/mail/username: pread(size=#### offset=########) failed: Input/output error
Editar “/usr/lib/systemd/system/dovecot.service”
agregar:
[Service]
…..
ProtectSystem=false
Fuente: https://forum.iredmail.org/topic13321-dovecot-error-open-failed-readonly-file-system.html