SBS2011 email receiving issues error #4.4.7 SMTP; 451 4.7.0 Timeout waiting for client input> #SMTP#

The issue might be due to anti-virus or similar scanning mechanism.
There is no information that could point to an issue where you are looking from the point of view of the server.

So is the sender getting this NDR in their email when sending to the SBS 2011 server?
the first place to start troubleshooting is:
a.  download, install, and run the SBS 2011 BPA and fixing everything it finds
b. Run the Fix My Network wizard and fix everything it finds
c. go to www.testexchangeconnectivity.com and see what errors you get.

No antivirus. I have found outthe internet connection is FTTC.
Running BPA

Test the issue from a remote location sending to your own.  You may have a provider provided SMTP proxy/filtering setup that is impacting messages larger than a certain size.
i.e. from a remote location point your outgoing SMTP to the SBS smtp server.
Then send an email with an attachment of 100k, 1m, 10m, 20m
On the client, you will see which emails go through and which do not.
 
If the client side indicates all messages went through, check the server for connections from your IP.

Does the server reflect connections from multiple IPs or are the connections from a single IP or IP range?

We have used another methord to deliver our emails.