Up through SimpleSync v4.7, connections syncing to an Ex200x destination use a default mailnickname mapping  reading:

mailnickname#64=~mail~

This mappings uses the source email address value to set the mailnickname attribute of each contact created on the destination.

As of Exchange 2007, the @ character is no longer allowed in the mailnickname attribute. Previously synced contacts will contain this 'invalid' character, since all synced contacts contain an email address in that value.

The next version of SimpleSync (v4.8)  will have a modified mailnickname mapping that uses ~smtpname~.~smtphost~ instead of ~mail~.   ~smtpname~.~smtphost~  is the same value as mail, except uses a period instead of an @ character. Therefor, the next sync run after upgrading to Ex2007 will fix the mailnickname value of every contact synced previously.

In the mean time, using the current version of SimpleSync, this problem is easily remedied with a custom mapping to assign a new mailnickname mapping.

Create a Custom Map File and modify the existing mailnickname mapping to read as follows:

mailnickname#64=~smtpname~.~smtphost~

Click SAVE. Click Apply.

On your next sync run, all contacts will be Modified, changing the mailnickname to an Ex2007 accepted value.