- Figure out a way to read the attachment data from the "different platform". Typically you will need to first authenticate and then make HTTPS requestsHTTP or HTTPS requests to get the data. Apex includes the building blocks you need for this but the details depend on exactly what the "different platform" offers.
- Use the Apex
Messaging.SingleEmailMessage
andMessaging.EmailFileAttachment
classes to send the email addresses. There is an example of that sort of code in the Creating an Email Attachment documentation.