I used the following command in PowerShell to test the users ActiveSync configuration status.
Test-ActiveSyncConnectivity -MailboxCredential (Get-Credential domain\username) -UseAutodiscoverForClientAccessServer
The command failed.
I then piped the output to a format list function to provide more details about the error received.
The error received was as follows:
"[System.Net.WebException]: The remote server returned an error: (500) Internal Server Error."
As I knew the problem was not to do with the mailbox contents as I had recently recreated the users mailbox, and this user was the only user effected by the issue, it had to be a problem with the Active Directory user account itself.
After investigating the user account I noticed the account was no longer inheriting permissions in Active Directory. After re-enabling inheritable permissions, this resolved the problem.