25/TCP is, technically, for MTA <-> MTA traffic, so many (residential, in particular) ISPs block it because "you shouldn't be running servers". 587/TCP is the

25/TCP is, technically, for MTA <-> MTA traffic, so many (residential, in particular) ISPs block it because "you shouldn't be running servers". 587/TCP is the "submission" port and is exactly what people should be using instead of 25/TCP (it usually -- but not always -- requires authentication and is pretty much never blocked). Indeed, that's the intended purpose!