Cannon you plonker!
Customer:  "Hi Pete, can you arrange for the mail to be put back on the original IP?"  Me:  "Sure, hey I know, lets check it's working first?"  Customer:  "Good idea."  Me:  "Nope .66 .69 or .71 is not accepting telnet connections."  Customer:  "That's strange? I'm sure it was .71 tbh."  Me:  "Oh"  Customer:  "What?"  Me:  "Helps if you put the port number on doesn't it?"  Customer:  "Just a bit."