This is a really useful thing for me to know, thanks for posting this. > This is the correct way of handling. As per the RFC, error codes win > the 400 range are "transient errors", meaning that they are temporary, > and the sender should retry eventually; while codes in the 500 range > are "permanent errors". Of course, servers are not required to adhere > to the RFC, but I haven't come across one that does not in this > regard. It's for their own benefit, for they stand to break the > entire system and lose e-mail if they send the wrong response. > > "Greylisting" and other anti-spam techniques should result in a 4.x.x > error. Any mail sending application should be smart enough to re- > queue these. 5.x.x errors must not be re-tried without > modifications. In my mail queue server I have a simple algorithm to > attempt to determine the reason for the failure, and sort them so that > they can be manually analysed and corrected if necessary. The most > common errors I've seen are: > 1. User not found (account closed) > 2. Destination server blacklisted in ORDB > 3. Common typo in domain name (e.g. yaho.com, gmail.cmo, etc.) > > dZ. >
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