Is it possible to fake incoming IP?

IP spoofing is the term used for forging an IP address, however it's very hard to accomplish with a SMTP connection as it requires an established TCP connection. IP address spoofing works as an attack when you don't need traffic coming back to the sending host. Using an analogy, if I send a letter to an address with a return address other than mine I will not get any response - it will be sent to the return address on the letter. I would not be able to form a correspondence with anyone if I'm forging the return address.

SMTP uses TCP, a protocol that establishes a connection between two hosts and controls the flow of information between them. A system trying to establish a connection using a spoofed IP address will not be able to set up a TCP connection as packets from other host will be routed to a the server which actually owns the IP address.

If someone was able to insert themselves in a point in the network between the two authentic servers they could use Network Address Translation to change the source IP address of packets from a server they control and set up a TCP connection, however this is very sophisticated and extremely unlikely. In any case you could defeat this by using public/private key pairs to verify the sending host. You don't need to be concerned about someone spoofing the IP address of your email server.