WA UE was never intended to be used as a large volume mailer. There are numerous potential issues on the server side that can occur that WA would have zero ability to deal with.
In the effort to combat SPAM, server admins typically throttle or limit the amount of email that can be blasted out from a domain at any one moment, in addition they may have a threshold number... Lets say 300 messages just as an example, at which point when that number is reached the server blocks everything else from going outward. Resulting in lets say a server timeout.
WA UE is awesome for sending out small to moderate amounts of messages smoothly and easily, but for large volume (>100+) addresses at once, you'll probably encounter server settings that will restrict your activities.