If formula that can display value from one of two fields depending on the contents of the fields

I'm attempting to write a formula that seems like it should be simple, but I cannot get it to work.

In Field C (text formula field), I need a formula that I presume will use an If statement.

If Field A contains "xxx@xxxx.com", display contents of Field B
Otherwise/Else show contents of field A in field C.

(the "xxx@xxx.com" email address trigger will always be the same email address.)

Any help would be appreciated.  Thanks.