Unless I've missed my guess, the problem isn't with your mailserver (you shouldn't be getting rejected by sendmail on localhost, so it seems it's not running there). I'm assuming you haven't changed Sendmail.pm per the POD documentation for the module. Sendmail.pm uses SMTP, and so it needs an SMTP server. The default server list is qw( localhost smtp.site1.csi.com ). I haven't a clue who manages the latter, but I'd imagine they pretty quickly became fed up with every user of Mail::Sendmail trying to bounce messages through them. I suspect the 550 you see is not from your SMTP server, but smtp.site1.csi.com. So, what you need to do is change the line in Sendmail.pm to point to your SMTP server. This may not seem kosher, but it's recommended by the docs. The doc also gives a couple of alternatives to modifying the module, in case you anticipate moving boxes and forgeting you did it, for instance.