It appears that the server has sendmail configured with a 'smart host' which it forwards all mail to.
I am not familiar with Gentoo, so I have to be somewhat generic here.
Sendmail it self uses a file called sendmail.cf for it's configuration. This file is often generated from a file called sendmail.mc.
You need to locate these files.
If sendmail.mc exists...
Edit it and find a line that contains the text 'SMART_HOST'. You should find that it refers to the old host name.
Change it to the correct host name and save the file.
Now you need to regenerate the sendmail.cf file which typically lives in /etc or /etc/mail. Assuming it is in /etc, the following command should work
m4 pathtosendmail.mc > /etc/sendmail.cf
restart the sendmail process
If sendmail.mc does not exist and only sendmail.cf is found...
find the line that starts with DS
You should find that after the DS is the old host name. replace it with the new host name and save the file
Beware that you can simply change the sendmail.cf even if you have a sendmail.mc, but if someone or some process comes along an rebuilds from the .mc file, any changes to sendmail.cf would be lost so it is better to change it at the source.
I hope this helps
My goal is not to catch you a fish and feed you for a day.
My goal is to teach you how to fish and feed you for a lifetime.
Since I hate fishing and love teaching, that works out pretty well.