Link Controller - Create Link(ISP) object

In case of a standalone system SNAT automap should do it automatically. Indeed there were changes in behaviour (means a SNAT address [if configured] needs to belong to the same traffic-group as the virtual server). With SNAT automap a self IP will be picked and in case there is an available floating self IP on the egress interface the rule above applies.

 

To be on the very safe side you could put your both "external" self IPs into a SNATpool and associate it with your virtual server.

 

An iRule could be used to apply service specific selective SNAT. I.e. you provide 2 virtual servers for inbound mail and want to make sure outgoing mail will be source NATed with exactly these addresses (makes sense as the external receiving MTA may do a reverse lookup to validate the authenticity). For this enhancement you can still go ahead without an iRule. Just create a new network wildcard virtual server on the internal interface but set the service port i.e. to tcp/25 and use a SNATpool containing the IP addresses corresponding to your MX records and the default gateway pool.