As Barry suggested on another thread you posted, the Astaro works best when the primary address on your External interface is the IP in your MX record. Another mis-step newbies often make is to skip the step of giving the Astaro a "proper" hostname. Instead of naming it "My new Astaro" or some such, it's important to use an FQDN that's defined in your public authoritative name server.
If you can't change the primary address on the External interface to .213, then you'll need a NAT rule:
Traffic Source: Any
Traffic Service: SMTP
Traffic Destination: External [{your .213 Additional Address}] (Address)
NAT mode: DNAT (Destination)
Destination: External (Address)
Destination Service: {blank!}
Cheers - Bob