List Spam Protection

The spam protection system we use at USD only applies partially to your mailing lists. Because the current system does not create quarantine accounts for list addresses, it will not quarantine spam messages. Messages that would be quarantined going to a standard MySanDiego address will instead be let through when going to a mailing list address.

Messages sent to mailing lists are subject to greylisting, however. Normally this should be completely invisible to you and to your subscribers.

Technical info

Greylisting works by remembering what servers have contacted us in the past. The first time a sending server tries to send mail to any USD address, the system bounces that message with a 4xx error message (as I write this, the error we give is 451).

When a sending mail server receives a 4xx error, it is required by the Internet mail standard to try again later. The sending individual should never see that bounce. When the sending mail server tries again with that message, it will be added to our list of “good” servers.

If, however, one of your members reports a bounce, and in the copied text of the bounce it reports a 4xx error from our mail server, that’s indicative of a problem either with their server setup or with ours. Please forward such error messages to us.

So far, we’ve only seen this happen once, though it may have happened other times and not been reported to us.