i did it via an own domain with destination number in the user part, e.g. +email@example.com
our mailserver forwards mails to that domain to my sms server where the mail is forwarded to a script which builds the file for sms server tools. this is the hardest part, because you need to consider multipart messages, mime-coded things etc.
i never had the time to complete my script, so mails need to be sent without mime-parts, which is done by sending mails without a body (message is in subject).
at this point it would be possible to create the necessary apoi-request to have playsms involved.