After loosing access to my server for several days due to spam using all the cpu on my server, I decided to make a few changes to limit that. It is a debian 10 server with Postfix version 3.4.7. One of the change I did seems to prevent me from sending mail using Thunderbird. In my /etc/postfix/mail.cf, I added these lines based on the postfix documentation to limit zombie spambot: smtpd_client_restrictions = permit_mynetworks, permit_sasl_authenticated, sleep 1, reject_unauth_pipelining smtpd_delay_reject = no I can still receive mail properly with Thunderbird. I am still receiving mail from other mail servers too so this looks like it only affects me sending mail (even to another mail on my own server) using Thunderbird so far. In the mail log, I see this: … postfix/smtpd: connect from unknown[…] … postfix/smtpd: NOQUEUE: reject: CONNECT from unknown[…]: 554 5.7.1 <unknown[…]>: Client host rejected: Access denied; proto=SMTP … postfix/smtpd: disconnect from unknown[…] quit=1 commands=1 Any suggestions on a way to fix this? If I only put this in /etc/postfix/mail.cf, I can send mail using Thunderbird but this is removing the zombie spambot protection: smtpd_client_restrictions = permit_mynetworks, permit_sasl_authenticated