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., gmail.cmo, etc.)
>       dZ.

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