[Exim] Forward file and SMTP authentication

I am running version 4.24 of exim, and I am using a forward file to forward
out of hours email alerts for certain email accounts to an email to sms
gateway.

I want to change to a different email to sms gateway supplier that uses SMTP
authentication to accept emails.

I have read the docs but I am still at a bit of a loss where to begin.

Will I have to write a specific router for this or could it be done inside
the forward file.

any help would be much appreciated.

tia

garry crothers