Actually what I would like is the possibility to put the routing rules on the delivery. It would be a turning point and a significant increase in functionality.
Imagine that a series of rules can be inserted into the delivery queue and applied in order of priority such as:
1) if the recipient is @hotmail.com use smtp2go gateway - continue with other rules
2) if the recipient is gmail.com use outbound ipv4 xxx.xxx.xxx - continue with other rules
3) if the recipient originates from a forward or an alias use smtp2go gateway - stop further rules
4) if the email is intended for more than 2 recipients use outbound ipv4 xxx.x..x.x - continue with other rules.
I repeat, in my opinion sm would make a significant leap in quality and would greatly reduce the need to use an exit gateway such as postfix on which to then apply the rules. That is, it would make sm independent. Furthermore, only SM could apply rules like 3
I hope to generate discussion and consideration of what has been said.
I understand it's in beta now and now is not the time. But once the beta is over I think this could be a great evolution.
edit...
add that the routing rules are present, with various levels of functionality, in
postfix
exchange
And these are just the ones I'm sure of.