1. Your message is too large to send. To send it, make the message smaller, for example, by removing attachments.
2. Remote Server returned '< #5.2.3 smtp;550 5.2.3 RESOLVER.RST.SendSizeLimit.Org; message too large for this organization>'
The fix for me was to run the following command in the Exchange Management Shell:
>Set-TransportConfig -ExternalDsnMaxMessageAttachSize 100MB -InternalDsnMaxMessageAttachSize 100MB -MaxReceiveSize 100MB -MaxSendSize 100MB
Use Get-TransportConfig to verify your settings.