Force Postfix to read/use your hosts file

By default postfix use DNS to resolv names, if by any chance you need postfix to use your hosts file, you will need to configure the smtp_host_lookup option to you main.cf.

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 !