I've got message from groups.io that mail sent to me is bouncing. Followed the link to check it out.
In Recent Bounces I found out list of bounce notification from several dates all with similar message like this one:
126.96.36.199: 550 Please turn on SMTP Authentication in your mail client. web01.groups.io [188.8.131.52]:53022 is not permitted to relay through this server without authentication.
I checked logs of my mail server and found out no traces of these connections and bounces.
Then I realized that it seems messages are bounced by mail server at 184.108.40.206. That is IP of server where my domain is hosted. Server does have mail server but for this specific domain, MX record is set for different address, meaning mail server at 220.127.116.11 is not used but other one, as specified in MX for this domain.
Usually what this means is that we had a problem looking up the MX record for your domain and fell back on using the A record. I don't know why we had a problem with the lookup, but it appears to be temporary.