Sending email pictures to to xxxx@vzwpix.com results in Failure Notice

Remote host said: 550 Message refused
is the error it usually says

but it only seems to be doing it w/ @vzwpix.com

And usually only w/ my Yahoo account, although gmail sometimes returns errors like this

Technical details of permanent failure:
PERM_FAILURE: SMTP Error (state 16): 550 SMTP connection refused

is this happening for anyone else too? it seems like a 50/50 chance that the message will go through.

iMac Mac OS X (10.4.10)