This is what happens when fetchmail pulls emails (a line from /var/log/maillog):
sm-mta: kAC0K9it002354: ruleset=check_mail, arg1=<email@example.com>, relay=localhost [127.0.0.1],
reject=451 4.1.8 Domain of sender address firstname.lastname@example.org does not resolve
Everything worked fine, until I had to go on a trip. Then this started to happen.
I solved it by adding this to sendmail.cf (actually, added it to sendmail.mc, but sendmailers over there know what I'm talking about):
My laptop is firewalled, so I won't get any spam or such. So this solved my problem.
But, does anybody know just what have happened here? I can dig those mxs, why sendmail can't? Just so you know, I've done my share of RTFMing, but I haven't find definite answer to what's going on.