Not sure why this is an error at all.
Yes, I read technote #1260359 entitled:
Forwarding mail to another address prevents busytime from recording Calendar data
In a mixed environment of Lotus and Non-Lotus users this is a common occurrence. Yes, there are some ways around it, but the simplest way is to just add the forwarding email and let all mail relay internally.
Now, if you enable DDM(Domino Domain Monitoring) which hopefully you do and then set the events you wish to track and be notified about through email, logging, database or whatever way, this will drive you insane.
Every time you start a server these "errors" are generated. In a small group or server you may not care, but with 100's or 1,000's of users this will start causing all kinds of issues. Admins will stop monitoring it, and you can exclude it by being more specific in what notifications you do want to receive.
However, when at a new client, who isn't using DDM, I like to run it and see what I get. Needless to say if email notifications go out you have swamped or crashed a mail router or at least delayed a bunch of important emails.
So I posted this on Ideajam to encourage IBM to adjust this one error accordingly.