Here is a diagram with the different steps involved in this scenario.
First, start by designing your attachment. See this article . This allows you to have the files attached to an email, even if they are not hosted on the execution instance.
You can send emails via a SOAP message trigger. For more information on SOAP requests, see Event description . In the SOAP call, there is a URL parameter (attachmentURL).
When designing your email, click on Attachment . In the Attachment definition screen, enter the SOAP attachment parameter:
<%= rtEvent.ctx.attachementUrl %>
When the message is processing/deploying, the system will get the file from the remote location (third-party server) and attach it to the individual message.
Since this parameter can be a variable, it should accept the fully formed remote URL variable of your file, sent via the SOAP call.