One of the queries we occasionally get from our customers is “how big an email can I receive?”. Every mail server has an upper limit on the size of an email it will accept, partly to protect itself from running out of disk space, and partly to protect the recipient from having to download an enormous, potentially unwanted, email.
However, this maximum size isn’t as clear-cut as it first seems. For example, if the maximum message size is set to 1MB, an email with a attachment smaller than 1MB may not be accepted.
One cause of this is that the message itself has a size, and it will also have headers detailing its route from the sender to the recipient. These are both counted in the message size, leaving less available for attachments.
But the biggest cause is BASE64 encoding which inflates the attachment’s size by a seventh. Without going into too much technical detail, this is needed because of the way that internet mail was defined many years ago. Hence a 0.9MB file becomes 1.02MB when BASE64-encoded and the mail falls foul of the limit.