The error is from the remote server not from yours.

What are your server settings and the domain settings?

The error indicates the email was over 10MB in size :

552 Message size exceeds maximum allowed size of 10485760

So if the email is truly 5MB, there is something up with the receiving email server.

*NOTE* with encoding, a 10MB file attachment is roughly over 10MB by the time it is sent.

-dave

As David pointed out, depending on how the attachment is encoded in the email, it will drastically increase the size of the email.  Commonly base64 is used for transfer-encoding attachments which adds on average 33% to the size.

Thank you for your support.  These are the Smarter Mail settings.Smarter Mail settings.png

Also, just wanted to point here, that I sent an email with 7 MB attachment to Gmail from my smarter email.  It went through, however; when I send email from Gmail back to my smarter email, it bounces back.  So all in all, there is some setting that is missing here.  Will appreciate help on this.

Thanks again.