problem with mail server

[rout3rx]

HI
i have number of plesk server, one of them works with all of the world mail services, many company!
sometimes thay don't receive mails from others! and sometimes customers send undelivered mail reasons to us:
like this:

Technical details of permanent failure:

Google tried to deliver your message, but it was rejected by the server for the recipient domain iasco.ir by mail.iasco.ir. [144.76.245.37].



The error that the other server returned was:

550 5.7.1 Command rejected

but not always this!
how can i solve this problem on my mail service for all of the time?
my mail server is: mail.iasco.ir
thanks

 

[IgorG]

Check this article - http://kb.parallels.com/766