It seems that it's a common spammers trick to use different ISPs for in and outbound traffic. Whether that is or isn't true, the previous settings always worked up until this recent episode.
It is quite common in the US for ISPs to block traffic on Port 25 that is not directed to that ISP's SMTP server. Exceptions are made for SMTP servers operating on ports 465 and 587 which require authentication (userid and password) to accept traffic. The intent of is to prevent the ISP's networks from being used for the distribution of SPAM and Malware.
How effective these tactics are I cannot say, but often they were and are instituted as a partial settlement in lawsuits brought against the ISPs by various state and federal Attorneys General.
