Ok this makes sense.
FYI, SendEx does not return a status if an email was send or not.
Only o.LastStatus is important. The code 221 means, the email was successfully send. Can you please check if this code is also returned on your XP machine?
Ok, this indicates, that the email was successfully send from the SimpleMail side to the EmailServer.
So the issue does not seem to be in SimpleMail.
Please also read the property "o.LastStatusMessage". Maybe it contains something more which could help us.