Duplicate "From" field in outgoing email

If I send an email using Outlook 2011 for Mac, it puts two "From:" fields in the headers of this mail.

If for instance I send a mail to myself, the source contains 

   "From: Mark Timmer <*** Email address is removed for privacy ***>"

and a bit further on 

  

   "From: *** Email address is removed for privacy ***". 

If I use webmail, this is not the case. Now, several mail servers reject my emails, giving a "permanent fatal error" with the error message

   "reason: 550 Messages should have one or no From headers, not 2.".

Could this please be resolved? I doesn't seem like I can change anything to fix this, can I?

Kind regards,

Mark Timmer