iRule for SMTP: Passing Client IP Addr to backend mail servers

This is pretty nice.

CLIENT_DATA and SERVER_DATA are not going to trigger on their own. You have to initiate TCP::collect in the CLIENT_ACCEPTED or SERVER_CONNECTED. That said, you don't need these events.

Also, "event STREAM_MATCHED enable" is not needed. When there is a match, this event will fire.

Simplify the code under STREAM_MATCHED for easier troubleshooting:

when STREAM_MATCHED {
    set mstring [STREAM::match]
    log local0. "STREAM_MATCHED: string: $mstring"
    set replacment [string range $mstring 0 1]
    append replacment "LO $c-addr\r\n"
    log local0. "STREAM_MATCHED: replacement string: $replacement"
    STREAM::replace "$mstring/$replacment"
}

Don't forget the generic (empty) STREAM profile to be added to the virtual. Here is a good link for more info: https://devcentral.f5.com/articles/ltm-stream-profile-multiple-replacements-regular-expressions.U3i7UygfSok

Finally, why not test this using a mocked up SMTP server. Setup up an echo server and use telnet for a client. See if the BigIP does the replacement.