SMTP Sniffer, Search and replace content

I have 2 program that communicate using SMTP, both working on the same machine, one send emails and the other have build in SMTP server In order to solve some problem I need to do a simple search and replace on the email message text (like replace the string ABC with 123) I CANNOT configure the port that is used for SMTP in any of the programs, so I cannot just do a SMTP bridge. I need a program that will be able to do sniffing on local (LOCALHOST) sockets on SMTP port and be able to search and replace the data on the socket. ## Deliverables * Must be written in C# * Need to have easy install program * Need to deliver source code * should be fast and not process any other sockets on the machine