In past 2 weeks, i got emails sending problem.

All my zimbra servers verison is Release 8.0.2_GA_5569.RHEL6_64_20121210115059
Server no.1 (mail.mydomain.com located in Thailand ISP) got problem that it can't send a big size email (over about 5MB.) to @yahoo.com (email size below 5MB. was sent fine)

Error Log :
dsn=4.4.2, status=deferred (lost connection with mta6.am0.yahoodns.net[] while sending message body)

so I've set a new zimbra just for MTA server in USA (mta.mydomain.com) server no.2

and set zimbraMtaRelayHost on server mail.mydomain.com to mta.mydomain.com , Now can send emails to yahoo.com any size
BUT server no.2 (mta.mydomain.com) have problems sending emails to some domains that mail servers located in Thailand too. (most of errors are timeout errors)

Is it possible that zimbra/postfix relay a mta host for only some recipient's domains?
example :
@yahoo.com @yahoo.co.uk @yahoo.co.th relay to mta.mydomain.com
@gmail.com @hotmail.com relay to mta2.mydomain.com
other recipient's domains send directly from server no.1 mail.mydomain.com

Thank you very much.