Hi,
I have not had this problem previously.  Trying to a mailing to a subset of my subscribers - about 7,000 I got this error message:
fsockopen(): unable to connect to mydomain:25 in /myppathtolistmail/admin.php on line 765
could not open socket with fsockopen()
Also I got an error at the bottom of the send screen that said, 
Server Said:
[configuration] 
With a  button, when I clicked on the button the error above
showed up.
What's going on here?   Any ideas on how to fix it?
Thank you,
-Brett