Guys,

I setup Outlook for Mac (2011) for a client and added a signature (.jpg) which is essentially her company logo etc. New email account is Microsoft Exchange account. Customer's device is a fairly new Macbook Pro. 

When I sent a test message to my Hotmail a/c, everything looked great on Outlook.com. I also sent the email to my business email account and viewed the email in Outlook 2007 and everything looked great.

The problem occurs when the email is viewed on an iPhone or Android phone device. The signature appears ok but the email subject is appended with the CID:xxxxx-xxxxx-xxxxx-xxxxx-xxxxx string mentioned in the heading. 

For testing, I decided to load up the same account and signature in her Mac Mail program and no issues at all. Email looked fine on all devices including iPhones and Android phones and the CID: string was not present. Problem is her Mac Mail program already has 3 personal email accounts in it so we were wanting to keep the business account separate in Outlook. Also, for exchange accounts, Outlook would be preferable to the Mac Mail program (although my dealings with Outlook for Mac 2011 over the years has proven to me it is far inferior to the Windows version, database is susceptible to corruption and less feature-rich etc).

I installed the latest update which I think was 14.7.

Can anything be done about this? I have Googled it for ages and there appears to be no documented solution or work-around. 

I did find this : Web Link (view comments at bottom) which suggests I might be able to edit the com.microsoft.Outlook.plist file. Although, I am not really keen on the idea of setting up the signature as a url link because so many email hosts these days block links as they are perceived to be spam.

P.S. I checked in the settings in Outlook and confirmed that Outlook for Mac 2011 is setup to send emails in HTML mode.