511 sorry, can't find a valid MX for sender domain

[Fear37]

Mar 9 16:08:37 pmg pmg-smtp-filter[13257]: A17EA5E66CC7544860: accept mail to <achessa@ourdomain> (B184F21DB5) (rule: default-accept)
Mar 9 16:08:37 pmg pmg-smtp-filter[13257]: A17EA5E66CC7544860: processing time: 0.462 seconds (0.42, 0.016, 0)
Mar 9 16:08:37 pmg postfix/lmtp[13835]: 5963D21DB4: to=<achessa@ourdomain>, relay=127.0.0.1[127.0.0.1]:10024, delay=1.4, delays=0.98/0/0/0.47, dsn=2.5.0, status=sent (250 2.5.0 OK (A17EA5E66CC7544860))
Mar 9 16:08:37 pmg postfix/qmgr[13415]: 5963D21DB4: removed
Mar 9 16:08:38 pmg postfix/smtp[13864]: B184F21DB5: to=<achessa@ourdomain>, relay=ourmailserver's IP[ourmailserver's IP]:25, delay=1.2, delays=0.02/0/0.15/1, dsn=5.0.0, status=bounced (host ourmailserver's IP[ourmailserver's IP] said: 511 sorry, can't find a valid MX for sender domain (#5.1.1 - chkuser) (in reply to MAIL FROM command))
Mar 9 16:08:39 pmg postfix/qmgr[13415]: B184F21DB5: removed
Mar 9 16:11:00 pmg postfix/smtpd[13830]: disconnect from rs227.mailgun.us[209.61.151.227] ehlo=1 mail=1 rcpt=1 data=1 quit=1 commands=5

This error sounds like it's our mail server rejecting the email from pmg, however our users said they were getting those emails before we implemented pmg, and our mail server has not been changed at all.

Any help would be much appreciated.

 

[dcsapak]

that is definitely a message from 'ourmailserver's IP[ourmailserver's IP]:25'

however our users said they were getting those emails before we implemented pmg, and our mail server has not been changed at all.

but maybe the sender domain changed somehow?

 

[Fear37]

but maybe the sender domain changed somehow?

It's possible, I'll give them a call. Thank you