Add mail server to Proxmox Spam Gateway Server&action=submit

At Proxmox Side map the Domain

Under configuration tab on left 
Mail Proxy --> Relay Domains --> create --> add domain

then

Choose Transport and enter 
domain , Ip or name of the zimbra server (e.g mail.heromotocorp.biz) ,port as 2525 , uncheck Mx

Now allow the mail server to send the mails via proxmox via port 26

choose Network 
create by puting the IP address/32 and comment 

At zimbra Server end add following line in right master.cf.in ( in case of zimbra 8.7+ /opt/zimbra/common/conf/master.cf.in )

2525      inet  n       -       n       -       1       postscreen

after line showing

smtp      inet  n       -       n       -       1       postscreen

Save the file and

Also change the outgoing SMTP relay server by as zimbra user

zmprov ms `zmhostname` zimbraMtaRelayHost 139.59.59.44:26

Restart zimbra of

zmmtactl restart

Now Point the MX record of concerning domain to gateway-1.tetrain.com which is proxmox mail gateway server