With Inbox.com, you can send and receive messages up to 20 megabytes (MB) in size. However, the precise amount allowable will depend on the attachment.
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When you add an attachment, the size of a file may increase because transport encodings are automatically added. (Transport encodings are the information that allows your message to be safely sent and read.)
This means that in some cases, attachments that are 15 to 20MB in size may push the total message size above 20MB. When this happens, Inbox.com displays a warning that your message exceeds the 20MB limit.