- Email remains in mail queue: Postfix tries to send many emails to the external recipient but only one email was sent initially
- Many following records appear in /var/log/maillog* files:
CONFIG_TEXT: postfix/smtpd: C7697151C60: client=mail-it0-f49.example.com[203.0.113.2], sasl_method=PLAIN, email@example.com
postfix/smtp: C7697151C60: to=<firstname.lastname@example.org>, relay=mx.spamexperts.com[203.0.113.2]:25, delay=38, delays=0.67/0/3.6/34, dsn=4.0.0, status=SOFTBOUNCE (host mx.spamexperts.com[203.0.113.1] said: 550 The sending IP (22.214.171.124) is listed on https://spamrl.com as a source of viruses. (in reply to end of DATA command))
IP address of the server got in blacklist https://spamrl.com due to outgoing spam and/or viruses being sent from it. Since another SMTP server rejects the email with soft-bounce status, Postfix tries to send it for 3 days with 5-15 minutes time interval according to default settings.
Find and exterminate the source of spam according to the article How to find the source of spam.
Remove IP address of the server from the blacklist https://spamrl.com according to the instructions on this website.