You’re getting an error that Outlook or Thunderbird cannot save to the sent folder and you’re connected to an imap server running dovecot. You’re also seeing Maximum Number of User+IP connections in
/var/log/maillog as people try to send email messages.
Please edit the following configuration to accept more simultaneous connections from a single IP. Add it if missing.
Add or edit the line and increase it.
mail_max_userip_connections = 50
Restart dovecot via
systemctl restart dovecot