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In the smaller window that appears, set a maximum size limit for messages that are sent and received, and then save the changes made.
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However, you can change the maximum message size for an individual mailbox with the Office 365 Admin Center. This is because email text is usually quite short and by having this default setting, users are restricted within the maximum attachment size that has been set. In Office 365, the default size for messages and email attachments is not set at its maximum capacity.
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