fixing the issue that some mail servers interpret "mail from: <abc@examp...

Some mail servers interpret "mail from: <abc@example.com>", "rcpt to: <abc@example.com>" as a relay request and respond with "Sender address rejected: User not allowed to relay via this host" even if abc@example.com exists. 

Making the "mail from" customizable, will solve this issue. The change is backward compatible.