#5.5.0 smtp;550 Recipient unknown error when sending from a secondary domain.

My exchange server is taking care of email for 4 domains, a.com, b.com, c.com and d.com. a.com is my primary domain that I set up when I first set up exchange. the other 3 domains, b c and d, are extra domains I've purchased and added in on the recipient policies to receive email on. This has worked great for me so far.

The problem comes now when one of my users needs to be able to send email from b.com rather than a.com. I went into his active directory user profile, and changed his primary email address from user@a.com to user@b.com. Every time they send email from that address to an outside address they receive the following error:

Subject: test
Sent: 5/9/2006 12:28 PM

The following recipient(s) could not be reached:

  outsideuser@outsidedomain.com on 5/9/2006 12:28 PM
  There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
  <a.com #5.5.0 smtp;550 <outsideuser@outsidedomain.com>, Recipient unknown>
I've also tested this with domains c.com and d.com, they have the same issue

I know that the MX records are set up correctly, I've checked them on dnsstuff.com. I also know that the address I am trying to send email to is correct.

I've tried using telnet to track down the problem (http://support.microsoft.com/default.aspx?scid=kb;EN-US;153119), but had no luck.

I saw a Microsoft article saying that it may be the outsidedomain.com's firewall, however I know this is not the case as email sent from a.com goes through with no problems

Lastly, the emails all work correctly internally, its just when I try to send over the internet that I receive the error.

Thanks in advance for the help!
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Is the second domain setup the same as the first domain?
I usually recommend that they are configured identically - same MX records etc, so that you don't have any problems with the reverse DNS lookups. If the server is announcing itself as domain.com and not host.domain.com then other sites can have issues with that.

Therefore the first thing I would suggest is to check your DNS is configured correctly.

Do you send email through a smart host? If so, you may want to check the smart host will allow the other domains to be sent through it.

bigft14CMAuthor Commented:
That didn't exactly help, but it pointed me in the right direction. I routed my smart host to relay mail through my ISP's mail server and now everything works.

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