I would assume it is a file size problem, but you posted in the printer support forum instead of the App Gallery forum where they may be able to help you from the server side. You should definitely be posting over yonder
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Many customers we have will scan a document to their email using the scan to cloud email app and it will work just fine. Then they will send another scan through the scan to cloud email app and it say COMPLETED on the printer but never arrive in their inbox. We checked with all IT personel and there is no Firewall blocking the emails. PLEASE HELP!!