maybe I spoke too soon. 4 mp3s went, and the next 2 came back with this:
Command: Data…
Response: 550 SMTP connection refused
Strange.
Think maybe I hit some obscure file transfer size limit? I tried sending it from my work email as well, same error.
EDIT: Curious! I sent one of the mp3s that worked the first time, and it went through successfully. What could be different about these files?

I know my phone limits the file size of ringtones and the length. Anything over 10 seconds can’t be used as a ringtone. I also remember it was a pain to find that out.
They also might be doing ID3 validation, so your files may need ID3 tags even if they’re empty.
I’m pretty sure they have ID3 tags, though I’ll have to check at home in order to tell for sure. They’re as large or smaller (file size wise) as the other files – since they’re all saved with the same hz/bits, I would expect them to be reasonably similar in length. I’ll check that when I can, though.
The thing is, I don’t even get the text message with these files. It seems to be rejecting the message -entirely-, rather than not allowing me to make it a ringtone.