Hi,

Does this user have a mailbox on Microsoft Exchange? If not, this could be the issue.

Regards

Yes, he's configured the same way as the other employees.

We had a similar issue.  User swore up and down they weren't receiving workflow emails.  Swore they had checked junk settings, etc.  When we actually went and looked, every email we had ever sent to them was sitting in their Junk folder.  Ah, the joys of end-user support. :)

To see what email K2 is sending notifications to, you can execute the Get User Details method on the UMUser SmartObject.  Format of the FQN parameter is <security label>:<domain><user>, e.g. K2:domainuser.

I just tried that SmartObject and the e-mail looks correct.

I was the same as you and figured it was in SPAM, but I had the IT team log in to his mailbox and confirm it wasn't there. They also searched the exchange server for the sender and recipient, but found no record of the email ever going out. 

These are all good suggestions, I'm just hoping there's something easy I overlooked.

Check the HostServer and AdumError logs in the K2 blackpearlHost ServerBin directory.

OK, now we are getting somewhere. There is an error in the HostServer log (below).

It would be helpful if there was more information though. In the SmartObject tester results, from earlier, the email address that is returned appears to be formatted properly. 

"187860604","2014-04-02 09:18:37","Error","MessageBus","61007","SendMessageException","MessageBusRuntime","61007 Permanent failure sending message RDC - Officer Approval Notification: System.FormatException: The specified string is not in the form required for an e-mail address.
at System.Net.Mail.MailAddressParser.ParseAddress(String data, Boolean expectMultipleAddresses, Int32& index)
at System.Net.Mail.MailAddress..ctor(String address, String displayName, Encoding displayNameEncoding)
at SourceCode.MessageBus.Imap.SmtpDestination.ToFullEmail(Dictionary`2 collection)
at SourceCode.MessageBus.Imap.SmtpDestination.Send(String title, MessageExtendedInformation extended, IEnumerable`1 attachments, MessageBodyReadere] messageBodies)
at SourceCode.MessageBus.Server.MessageBusRuntime.SourceCode.MessageBus.IMessageDestination.Send(String title, MessageExtendedInformation extended, IEnumerable`1 attachments, MessageBodyReadere] messageBodies)","","","DC-SERVER-3:C:Program Files (x86)K2 blackpearlHost ServerBin","187860604","06de311dc57d42f98e106e06ca9ca471",""

EDIT---

I'm testing to confirm, but it seems like if I do an actual Email Event he receives the email. If I just use the Default User Event Notification (like an infopath event) wizard, he does NOT. Is it possible that those 2 events use a different method of pulling the email address?

I wouldn't think so.  Everything in K2 tends to revolve around the identity cache.  So long as you are really doing the exact same users (e.g. jdoe vs jdoe2 or jdie), K2 should get the same email address.

Are you using the default security provider for AD?

It may be time to open a support ticket.

In case anyone runs in to the same issue, I was able to get this resolved with help from the Support team.
I ran the following query on the K2 database:

--Begin
Select Properties From K2]..Identity]..Identity]
Where FQN = 'K2::domain]]username]'
--End

and noticed that the XML result had an additional '<item name="" value="_scnull_" />' element that the other, working users did not have. I removed that element and the user began receiving emails.

As for what caused the issue. Support is still unsure but the Identity table gets populated anytime a user/group/role comes across in K2 for the first time i.e. Dest User in workflow, etc. K2 actually caches users/group information in this table so it does not have to query the security provider, such as AD, SharePoint, etc, each time they come across. The table is refreshed at a certain interval.

Keep an eye on this.  The Identity Cache refreshes itself periodically, and if something is borked somewhere else, the same problem could reproduce itself when the user's cache refreshes.

There is a discussion about managing cache refresh here.  If you need to force a refresh a particular user, you can run the following SQL on your K2 database:

update aIdentity].tIdentity] set
ExpireOn = '2013-01-01',
MembersExpireOn = '2013-01-01',
ContainersExpireOn = '2013-01-01'
where FQN='K2:domainuser';

Did you get solution for this?

Just wanted to note in addtion to already mentioned solution that it used to be a known issue with K2 Host Server Notifications in 4.6.5 which was resolved in 4.6.8:

"A Client event notification would fail when the URM received a property or a name without a value. The URM server requested data from Active Directory (AD), and AD would return the data, but if an error occurs in AD, a SCNULL would be returned and thus inserted into the database. A property in the Identity.Identity table would return something like: <item name="" value="_scnull_" />. This caused the associated XML not to be formed correctly, and thus the notification would not run."

See "K2 blackpearl 4.6.8 Resolved Issues List" for details.

So if you are running versions earlier than 4.6.8 you may need to check if affected user has <item name="" value="_scnull_" /> property in Identity.Identity table. As a workaround/quick fix you may remove this propery and for a long term solution you either have to upgrade to 4.6.8 or try to obtain a coldfix for your version through K2 support.