Hi @11545032/persistenttest-7723,
Hope you are doing well.
As per message-get and attachment-get we can retrieve content of message and attachment as shown below.
**For Messages**
GET /me/messages/{id}/$value
GET /users/{id | userPrincipalName}/messages/{id}/$value
GET /me/mailFolders/{id}/messages/{id}/$value
GET /users/{id | userPrincipalName}/mailFolders/{id}/messages/{id}/$value
**For Attachments**
GET /me/messages/{id}/attachments/{id}/$value
GET /users/{id | userPrincipalName}/messages/{id}/attachments/{id}/$value
Hope this helps.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".