5.1.0 - Unknown address error 550-'5.7.1 Requested action not taken: mailbox not available'

When I send an email from an outside source to one of my companies email address, I get an error message stating:

5.1.0 - Unknown address error 550-'5.7.1 Requested action not taken: mailbox not available'

help......
pollardb28Asked:
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ATPhatzCommented:
Do you have a web-shield set up? If so you might want to take a look into it.
Irwin SantosComputer Integration SpecialistCommented:
Is the mailbox actually existing??

It could be that the user of the mailbox, first needs to login to actually enable the mailbox
pollardb28Author Commented:
the mailbox does exist.....the user is able to receive emails from co-workers.  do not have a web shield set up
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Irwin SantosComputer Integration SpecialistCommented:
is it only from a particular address...how about send from ANOTHER email address?
pollardb28Author Commented:
i tried sending from 3 different email addresses and get the same error.....
ATPhatzCommented:
Have you looked at your smpt connectors?
pollardb28Author Commented:
yes i have it looks fine.....the strange thing is everyone gets external emails except for this one user.....
amaheshwariCommented:
Please follow these steps:
1. Exchange Server checks the SMTP addresses in the exceptions list to the block list service rules. To view this list, click Exceptions on the Connection Filtering tab of the Message Delivery Properties dialog box. If the sender's SMTP address appears in this list, Exchange Server bypasses the RBL.
2. Exchange Server checks the recipients that appear in the Recipients list on the Recipient Filtering tab of the Message Delivery Properties dialog box. If the message recipient appears in this list, Exchange Server returns the following error message to the sender:
550 5.7.1 Requested action not taken: mailbox not available

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