I use the above process on my server; I have the main instance of M32 that filters all the outgoing mail.

If  the "From" header = a.com, message is processed by the main instance.

If the "From" header = b.com, message is moved to the mbox b-out, where wsmtpex gets it, and forward it to the "B" instance of M32.

If the "From" header = c.com, message is moved to the mbox c-out, where wsmtpex gets it, and forward it to the "C" instance of M32.

Unfortunately there is a couple of cons (at least, here)

1) when the msg is moved to the b-out or c-out, the "From" header is removed.

2) it seems that wsmtpex does not support multiple recipients; so, if I send an email to - say - myfriend1@gmail.com and myfriend2@gmail.com, only the last receives it, because all the other addresses are lost.

 It may be a misconfiguration, but I couldn't get rid of this.

Best regards

Filippo

We currently use MercuryC to relay our mail through an outside company.

That company has implemented outbound spam filtering now.

Is it possible to have a second instance of Mercury, and do a conditional relay of outbound mail based on header information ?

 In other words, I'd like to send all mail out via MercuryC, except email with a FROM address of MYDOMAIN.com and a TO address of GMAIL.com - these I would like to send outbound directly (probably through a second instance of Mercury).

I haven't tested this but it should probably work to have an outgoing filtering rule move messages that match that criteria to a specific mailbox, and then have WSMTPEx (see http://community.pmail.com/forums/thread/22614.aspx) send everything in that mailbox on to the other Mercury instance.

/Rolf