Unsettled wrote:

But the actual solution is so hard to find -- can someone explain?

The solution is not hard to find, as you seem to have had no trouble finding
it, but the causes are varied. The most common problem is the connection to
the SMTP server timing out or being interrupted and this can often be caused
by using an antivirus scanner to scan outgoing mail, something that is never
of benefit.
--
Brian Tillman