Routing to multiple external SMTP servers

Not OOTB.

You'd need to persist with your original thoughts, and write an iRule to insert the credentials into the stream dependent on the member being selected.

You didn't elaborate why it didn't work previously...

H

Here is the iRule:

when CLIENT_ACCEPTED {
    STREAM::expression {@username@user123@ @password@pass123@}
    STREAM::enable
}

It works when replacing username string as a part of subject/body but not when it's the same string within the actual SMTP username or password.