Just in case this may be it. I use several different identities in Pegasus Mail to access completely different pop3 accounts but had accidentally ticked "checking multiple identities" under the "receiving pop3" Internet options. It did not matter when I used selective mail download but when I just hit download I had a similar effect ... just an idea.

Cheers

Thomas

I have three users.  Recently, some mail in the new mail for John started to appear in the new mail for David.  The messages have John's email address, but they now appear in David's new mail.

How does this happen?  Can I do anything about it?

This only happened once, about a week ago, so maybe it isn't a big problem yet. 
 

Not sure, how are you actually receiving the mail?  MercuryS via SMTP?  MercuryD via POP3?  Domain Mailbox?  I can see things like this happening if you are receiving via MercuryD using a "domain" mailbox and there is no header for the original RCPT TO: header.

POP3 mail from comcast.net is how I get my mail for all three users  I thought that all three were set up the same way, but apparently not, if you consider the other thread here. 

[quote user="dthomsen8"]POP3 mail from comcast.net is how I get my mail for all three users  I thought that all three were set up the same way, but apparently not, if you consider the other thread here. 
[/quote]

POP3 from separate POP3 accounts?  POP3 from a domain type POP3 account?  If this is a domain type account where you all use the same POP3 account this this can happen.  When they are separate POP3 accounts there generally is no interaction.

POP3 is from entirely independent Comcast accounts. 

Then I'm not all that sure what is happening.  Pegasus mail downloading mail from separate POP3 accounts into separate user accounts can never intermingle data, everything is coompletely separate.