Some emails are not syncing from Outlook.com to Outlook 2013

Outlook 2013, Windows 8 Mail App and my WIndows Phone 8 are connected to my Hotmail / Outlook.com account. 

99% of my emails sync without issue.  However I am getting email from a couple of contacts that when the new email comes in from them it will not sync to my Outlook 2013, Window 8 Mail App or my Windows 8 Phone.  If I reply to the emails that will not sync using Outlook.com, any replies they send back sync fine.

There is something about the initial email that get I from these two contacts that prevents it from syncing.  Maybe something in the signature icons / pictures they are using, or maybe something in the format. 

They are business emails, usually with a paragraph of text.

These contacts are important business contacts and missing an email from them since I do not get it on Outlook 2013, on my phone or in the Windows 8 Mail app is not acceptable.

Thanks in advance for any help you can give.

Rob.