Why does forwarding mail off the server hurt my server's mail reputation?

Question

Why does forwarding mail off the server hurt my server's mail reputation?

Answer

Forwarding all mail received by an address on your server to an address not on your server can hurt your server's mail reputation.  Anytime your server sends mail that did not originate on the server to another server, it's called "relaying".  You want to make sure that your server does not relay spam.  Relayed spam will look to other servers like your server is the one doing the spamming.  Additionally, forwarding legitimate mail (sometimes called ham) can break features like SPF, which can cause that legitimate mail to look like spam.  

You may also be interested in reading:

• How can I protect my server's reputation while forwarding mail?
• What alternatives exist to forwarding mail off the server?
• How can I get forwarded mail to pass an SPF check on the recipient's server?