Sending a message with a large attachment used to be the least worst option when it came to shipping data between people. In the Modern workplace where document and file sharing options are a lot better sending email with a large attachment can tends now to be the worst option especially when it comes to version control, expiration etc. However if you do find yourself with the need to send an email with large attachments (larger then 4MB) and you want to make use of the Microsft Graph API then it requires you use a different method from just sending the message in one post eg https://docs.microsoft.com/en-us/graph/outlook-large-attachments?tabs=http This does make sending a Message a lot more complex eg if you have a Message less the 4MB you can just make one REST Post to send that message. If you have a large attachment you then have a multi step process eg Create a Draft message where you set all the other message property eg Subject,To,Body Take the MessageId(graph item id) returne
Pushing the Envelope in Messaging and Office 365 Development