User gets banned (smtp)

I'm scratching my head over this one. Only one of many many users I have is getting blocked from sending emails by Fail2ban. I regularly have to unblock his (dynamic) IP address. Thinking it was a rogue email account in his mobile phone, I checked but found nothing wrong. So, invesitgating the logs I see...

(logs anonymised)
Feb 18 18:04:46 mail postfix/smtpd[18901]: connect from my_client_hostname[my_client_ip_address]
Feb 18 18:04:46 mail postfix/smtpd[18901]: warning: SASL...

User gets banned (smtp)