We'd have a recent spat of complaints from our Swirbo customers regarding their inability to receive mail from certain Google apps -- i.e. if you invite someone to view a blog, or docs.google.com document. Today I got an example of the actual error they are getting:
Technical details of permanent failure:
TEMP_FAILURE: SMTP Error (state 13): 450: Recipient
address rejected: Greylisted for 5 minutes
Anyone see a problem? The error we returned was 450, yet Google seems to think it was a permanent failure. Here's a bit from the SMTP RFC (2821):
4yz Transient Negative Completion reply
The command was not accepted, and the requested action did not
occur. However, the error condition is temporary and the action
may be requested again. The sender should return to the beginning
of the command sequence (if any). It is difficult to assign a
meaning to "transient" when two different sites (receiver- and
sender-SMTP agents) must agree on the interpretation. Each reply
in this category might have a different time value, but the SMTP
client is encouraged to try again. A rule of thumb to determine
whether a reply fits into the 4yz or the 5yz category (see below)
is that replies are 4yz if they can be successful if repeated
without any change in command form or in properties of the sender
or receiver (that is, the command is repeated identically and the
receiver does not put up a new implementation.)
SMTP 4xx errors are temporary and typically represent transient errors. An SMTP client, upon receiving an error like this, should (i.e. is encouraged to) try again. Greylisting is a spam-filtering technique that takes advantage of this fact by issuing temporary rejections to new clients, in effect filtering out mail sent by software not smart enough to retry (as any proper mailserver would). Google should be queueing and retrying these messages. Anyone else running into this?