Fix SmtpClient handling exceptions as timeouts

[MihaZupan]

This is a bug fix.

Only non-async calls on SmtpClient can timeout, but the flag is checked in the async path as well.

As the flag is not reset in the async path, it could already be set and SmtpClient would swallow any real exception and instead throw a timeout.

CC @dotnet/ncl

[stephentoub]

This is a bug fix.

What issue is this fixing? Can you share the issue number / link?

[MihaZupan]

To my knowledge, there isn't an issue opened about this, I just found the issue when reading the code.

Steps to reproduce would look like:

1. Use synchronous Send that times out
2. Use any async Send that fails for whatever reason (connection, security ..) - the exception will always be TimedOut (even tho async calls can't time out in SmtpClient)