Intention:
Redirect (reroute) specific email addresses via, for example, an SMTP service:

Steps:
– Enter the destination domain in the Advanced Routing Table (Email ==> Email Accounts/Email Routing)
– Enter the same destination domain in the (Email ==> Global Filters / Relay Recipients) as @domain

Example:
eg. rerouting all emails of destination domain mydomain.com via a relay server (relay.myserver.com)

(Email ==> Email Accounts / Email Routing)
Select ‘Add new Transport’
Fill in the fields:
Server: (Select the server)
Domain: mydomain.com
Type: (select smtp)
No MX lookup: (leave unchecked)
Destination: relay.myserver.com
Sort by: 1
Active: (Checked)

(Email ==> Global Filters / Relay Recipients)
Select: ‘Add new relay reciepient’

Fill in the fields:
Server: (Chose the server)
Relay recipient: @mydomain.com
Active: (Checked)