I hope no one uses this in production. Handling SMTP well is all about the edge cases, and this doesn't seem to handle any of them.
This is just a thin wrapper on python stdlib smtpd.SMTPServer, which is itself not production ready. It also doesn't really make it easier. The original API is better. Why are magic decorators better than inheritance again?
Also, monkey patching? Hidden side effects make sad pandas cry. Let me throw up a little in my mouth.
There are zero unit tests for this, and no functional tests.
The original API has better documentation. What are the to, sender, and body arguments?
Below seems a simpler API, and gives you the subject too. I thought humans would be interested in the subject... but I guess not.
import inbox
for m in inbox:
toprint = m.to, m.from, m.subject, m.body
print ("to:{} from:{} subject:{} body:{}".format(toprint))