Abdulkerim,

 

The Max. Message Size takes into account the entire message size: HEADERS + BODY + ATTACHMENTS which are then automatically MIME encoded (which in itself increases the size 35-40%). A Max Message Size of 7500 kb would roughly allow for only 27% overhead on an email with a 5482 kb attachment, meaning it isn't enough to allow for an attachment that size.

To account for all this overhead you would want to allow for a 3000 kb buffer on 7500 kb, so setting Max. Message Size at 10500 kb should effectively allow for attachments of 5000-6000 kb.