I don't understand how an external gateway for emergency use is practical. If SmarterMail is down, nothing is going out. Maybe I don't know enough. Very likely, actually. 

because you are imagining a scenario where your SM installation is broken.

But this is not the only case.

Imagine that your IP is rejected by some provider, sometimes because one of your users has done something bad, but sometimes also for invalid reasons.

Look at this case for example:

I assure you he is not the only one.

In this case an outbound gateway saves you

Ahh, I see.  We have been blacklisted before, causing 3 days in bounces from specific providers.  One switch would be useful.

bump.

Sorry if I bring up the topic again but I'm curious.

As I told you I was trying smtp2go and to verify the sender here is what it does to set on DNS

As you can see, the spf record is not modified

But then a message sent through them passes the spf check

How do they do it?

Perhaps they do some kind of conditional SPF with variables instead of static names, then the DNS values you input above factor into whether that overriding SPF check passes or not. I'm not familiar with it but that's my theory!

They send email out on a via a subdomain host on the primary domain you specify, and match the SPF and DKIM to "that subdomain host" that sends out.

We've used them for about 5 years now -- stellar service although there are some drawbacks to their UI for managing a large amount of domains.

Thank you

You can create a postfix for outbound with multiples IP's

in the master.cf you can create the macros with varios bounding Ip's

In the main.cf use the sender_dependent_default_transport_maps = randmap { macros names }

will be balance the delivery.