Is there a way to keep separate Sent folders in Gmail for each of your custom SMTP servers?
I already use labels to differentiate them by the way, but 2 Sent Mail folders would be ideal.
You can create a filter that looks at the from: field and applies a specific label. You'd then need labels for each (sent-mail1, sent-mail2, etc.).
The messages will still end up in "Sent Mail" as well, of course.
I do something similar. I have filters to have all messages where a specific e-mail is in the from: or to: get a label for that address.
If you are stuck with just using labels you can go into Settings > Labels and hide the system Sent Mail label to avoid going there by mistake.