Hi @ashan h ,
The attachment size limit that you set is not the actual value. If you want to send attachments with an actual size of 5MB, you need to adjust the attachment size limit to at least 6.65MB.
You could refer to:
https://learn.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits#receiving-and-sending-limits
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