Allow null return sender in mail command

We have had issues with a remote mail server bouncing messages we are trying to send to because the remote server is trying to verify our server. 

550-Callback setup failed while verifying <xxx@xxx.xxx>
550-(result of an earlier callout reused). 
550-The initial connection, or a HELO or MAIL FROM:<> command was 
550-rejected. Refusing MAIL FROM:<> does not help fight spam, disregards 
550-RFC requirements, and stops you from receiving standard bounce 
550-messages. This host does not accept mail from domains whose servers 
550-refuse bounces.

Checked RFC 2821 in 3.7 specifies that a null sender in the MAIL FROM command is valid.

Thanks