ReWrite recipient address on outbound mail

Hi everone, 

We have a solution, where the user send a mail to a SMS gateway including an API key or password. Now, I want to make a rule that add this information before the message is sent to SMS gateway. 

Example:

The user sends a mail to 12345678@sms.suresms.com, 

After the TransportRule or AddressRewrite the recipient should look like this

12345678.SecretKey@sms.suresms.com. 

We run on Exchange Online. I have tried to add TransportRule and replace header but that didnt seem to work. As far as I understand the rewriteaddress is not available on Exchange Online. 

Anybody have any idea how to complete this? 

Thanks

Should have been in Exchange Online, and I cannot move it :-(