The Sender header is described in the RFC here:
2.1.3. Header Field: Sender
Mailbox of message sender
Specifies the mailbox of the agent responsible for the actual
transmission of the message. Defined as standard by RFC 822.
Some mail servers may require that this header is set in order for a message to be accepted.
Ultimately this header should be set by the mail client or script that is sending the message, but there is an option in Exim to set this header from the sender value passed to Exim.
- Login to WHM as the root user
- Navigate to: Home »Service Configuration »Exim Configuration Manager
- Locate the following option and enable it:
Set SMTP Sender: headers [?]
(-f flag passed to sendmail) This will create “On behalf of” notices in Microsoft® Outlook, but it may also help track abuse of the mail system since recipients will see the SMTP login used to send each message.
- Click the blue Save button