I’ve figured this out for mailjet. You probably want to be editing exim4.conf.template
Under ‘begin authenticators’ I added
mailjet_login: driver = plaintext public_name = LOGIN client_send =:apiuser:apikey
Under ‘begin routers’ I added
send_via_mailjet: driver = manualroute domains = ! +local_domains transport = mailjet_smtp route_list = * in-v3.mailjet.com
// This sends all non-local domains through mailjet. You can add specific domains if you want. The route_list is the name of the mailjet server I’m sending through.
Under ‘begin transports’ I have
mailjet_smtp: driver = smtp port = 587 hosts_require_auth = * hosts_require_tls = *
That was pretty much it. Chang the port as neccessary. Put your real apiuser and apikey in between the colons