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NOTE: Microsoft has implemented Receiving and Sending Limits. Storage - User Mailbox, Shared Mailbox, Resource Mailbox
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Note: when you attach a file to a message, the file size of the attachment increases (it can increase as much as 30% in size when it is sent). Message send/receive size limit (including attachments) - Keep in mind that someone's mailbox on the Office 365 server will be prohibited from receiving messages if they exceed their maximum quota limit.