Sendmail randomly has trouble resolving domains

[ReverantGR]

Sendmail 8.18.1-lp156.370.4

So out of the blue, Sendmail will randomly fail to resolve a domain. This happens to both incoming and outgoing emails. Example:

2025-06-16T09:52:39.492987+03:00 www sendmail[1248]: 55G6qbMY001248: ruleset=check_mail, arg1=eorders@msystems.gr, relay=mail-westeuropeazon11022088.outbound.protection.outlook.com [52.101.71.88], reject=451 4.1.8 Domain of sender address eorders@msystems.gr does not resolve

Immediately try a manual lookup on the server and it is successful:
www:/root # host msystems.gr
msystems.gr has address 195.201.185.180
msystems.gr mail is handled by 0 msystems-gr.mail.protection.outlook.com.

When it happens to outgoing emails, I can login to the server, and issue “host -t mx domain.com” and get back a result. Immediately try to send the email again, and it fails again. So it’s not the target domain’s problem and it also happens with big sites (ie ups.com)

I have no idea what may be wrong and no clear path as to what to start looking for. This is totally random and has even happend with gmail addresses.

[nrickert]

I think it’s a timing issue.

I never had this problem before I switched to “NetworkManager” to manage the network. And now, using “NetworkManager”, I manually restart sendmail after the system is up and that seems to solve the problem.

systemctl restart sendmail