changing email sender domain how?

Hello,

i am using ispconfig3 for like 10 domain, and the server as a domain name isp.mydomain.com

now some of my user get email bounced because they are using mail.mydomain.com and the receiving end see isp.mydomain.com

i would like to change only the domain for email to mail.mydomain.com without affecting the machine domain name..

how i can do this?

thank you

 

i need to be able to change the Mailserver greeting ... how can i do this?