Why is the Graph API response payload size about 1/3 larger than the email attachment size

Question

Hi,

When I GET an email's attachments via Graph API using the postman GET call: https://graph.microsoft.com/v1.0/me/messages/{{messageid}}/attachments

the uncompressed size of the payload is consistently about 1/3 larger than the attachment size. E.g.

• An email with no contents and just a single 1Mb attachment has an uncompressed payload size of 1.33Mb
• An similar email with just a single 20Mb attachment has an uncompressed payload size of 26.67Mb

Do you know why the payload size is so much larger than the attachment size?

Can I rely on it being 1/3 larger than the attachment size?

The reason is we have an application where users can download attachments to a database via Graph API and I want to set a limit on the size of the attachment that can be downloaded. So if i want to allow up to 10Mb attachments should I check for a payload size up to 13.33Mb?

Answer 1

Because of Base64 encoding, as explained for example here: https://learn.microsoft.com/en-us/exchange/mail-flow/message-size-limits?view=exchserver-2019

Base64 encoding increases the size of the message by approximately 33%, so the value you specify should be approximately 33% larger than the actual message size you want enforced. For example, if you specify a maximum message size value of 64 MB, you can expect a realistic maximum message size of approximately 48 MB.