The earlier response also mentioned (if it wasn't already in the chest), however that is a bit of a moot point if you are getting errors trying to send it.
What was the full text of the error message as it usually gives a reason why it was undeliverable ?
What is your ISP ?
You may have to resort to sending it the conventional way by zipping and password protecting the sample and sending it to avast, however if this problem is at ISP level that may fail also.