I hadn't noticed this until now and it's an interesting problem with Postfix...
I have a few sites on my system. Two of the sites have a user name of "jim". So that makes user names like web1_jim and web2_jim.
The interesting part is all mail for web2_jim is going to web1_jim.
Now, what I THINK is happening is Postfix isn't doing a complete domain name check on the user names. I say this because both domain names are identical in size and start with the same word. Something like mashersock.com and masherhost.com.
So you'd have
So, like I said it seems to be matching the first "jim@masher" and deliverying mail there.