SASL PLAIN authentication failed

Hey guys I hope you can give me a hand on this because I don't know what to do.

I constantly get this type of messages log, I know that it means that I could not get authentication and it failed to log in in the email, but I know the IP, is from a client of mine, and It seems like one of his computers can't log in correctly and is trying to log in and it generates this log. I personally went to his company and checked all his computers and every single computer is working correctly on the emails.

What I do is, I whitelist his IP to prevent IP blocking but the problem is that when his IP changes than they need to call me to unblock it. Please can someone suggest a solution, probably there's one device that is logged in incorrectly but it could be a phone and it's really hard to keep track of every device in his company.

Here is the log.

Nov 26 12:21:04 vps postfix/smtpd[18783]: warning: unknown[x.x.x.x]: SASL PLAIN authentication failed:
Nov 26 12:21:10 vps postfix/smtpd[18783]: warning: unknown[x.x.x.x]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
Nov 26 12:21:16 vps postfix/smtpd[18783]: warning: unknown[x.x.x.x]: SASL PLAIN authentication failed: UGFzc3dvcmQ6
Nov 26 12:21:23 vps postfix/smtpd[18783]: warning: unknown[x.x.x.x]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
Nov 26 12:21:25 vps postfix/smtpd[18783]: warning: unknown[x.x.x.x]: SASL PLAIN authentication failed:

Please I need help on this.

you should use search on  the forum before posting
you can search by the word UGFzc3dvcmQ6