Intermittent send mail failures

[Steve Piercy]

Hello,

Mailsmith 2.3.1 (422) on Mac OS X 10.5.8.

When sending mail, Mailsmith intermittently returns the
following error.

================

Mail account “foo”, user ID “f...@bar.com”:

An error occurred while sending the message “Re: subject” to “mail.server.com”.

The server is in the process of shutting down or starting up,
and cannot service your request. Please contact your ISP or mail
server administrator for assistance. (Application error code 23325).

The following information was returned as part of a transaction
with the remote server; it may assist in diagnosing and
resolving the cause of the error:

421 4.7.0 smtp3.relay.mail.server.com Error: too many errors

================

I can use the mail provider's web interface to send mail, as
well as Thunderbird (all server and port settings are the same).

I contacted my mail provider, and they mentioned something about
caching on Mac OS X, but were not able to provide details of how
to clear said cache.

Often I get fed up, walk away for a couple of hours, come back
and try again, and it magically "fixes itself" and sends mail again.

I'd appreciate any tips on how to troubleshoot, isolate, and
resolve the issue.

--steve

--steve