Postfix : 451 Error in processing Number of messages exceeds maximum per connection

If you have deferred mails with the following reason :

 451 Error in processing Number of messages exceeds maximum per connection  

You might have an issue with your default_destination_recipient_limit option which is set to 50 by default.

In my case it was a bit more complicated, I had a custom transport for this destination (@hsbc.fr) with the destination_recipient_limit set to 5 and even with a setting of 2, I still had the same error coming from their servers...

The solution was to turn off on demand smtp connection caching, which is activated by default.

The postfix documentation says :
"Temporarily enable SMTP connection caching while a destination has a high volume of mail in the active queue. With SMTP connection caching, a connection is not closed immediately after completion of a mail transaction. Instead, the connection is kept open for up to $smtp_connection_cache_time_limit seconds. This allows connections to be reused for other deliveries, and can improve mail delivery performance."
http://www.postfix.org/postconf.5.html#smtp_connection_cache_on_demand

 "high volume of mail" is unfortunately not specified. In my case, I had to send about 6K mails, the  destination_recipient_limit was effective for this destination but Postfix reused the same SMTP connection which triggered HDBC mailserver limit.

Conclusion :

You can either disable SMTP connection caching globally in main.cf :

 smtp_connection_cache_on_demand = no  

Or by smtp transport in master.cf (NOT TESTED) :

 smtpslow unix  -    -    n    -    -    smtp  
 ...  
 ...  
  -o destination_recipient_limit=5  
  -o smtp_connection_cache_on_demand=off  

Hope that helps !