This answer was originally posted on DCIM Support by spezialist on 2020-01-21
Dear Trevor Rankin,
If you mean sending plain text messages (by DCE software terminology is Short Message Email (SMS)), then a detailed answer to your question can be found in the reply of Balaraman Jagadeesan from post Details of HTTP Post.
If you mean sending standard SMS messages through external GSM/GPRS modem, then it is impossible with DCE-server. The only solution in this case is the use of third-party e-mail to SMS gateway appliance or special e-mail to SMS service, which can be provided by your cellular operator (for example, this is how I do it). More details about this can be found, for example, from Ed Tarento reply to post Sending SMS from DCE.
I hope this helps you.