>How should the program know?

if we're talking about logs from our own applications that we have written, the program should know because we can write it in a way that it knows.

user-defined config should be verified before it is used. make a ping to port 25 to see if it works before you start using that config for actual operation. if it fails the verification step, that's not an error that needs to be logged.

Would it make sense to consider anything that prevents a process from completing it's intended function an error? It seems like this message would fall into that category and, as you pointed out, could result from a local fault as well.

SMTP clients are designed to try again with exponential backoff. If the final attempt fails and your email gets bounced, now that's an error. Until then, it's just a delay, business as usual.