In order to resolve this, ensure that the phone number type is marked as an email address. 

In Raiser's Edge, follow these steps:
  1. Click Config
  2. Next, click tables and select phone types from the list
  3. Double click the email address that is missing in OLX
  4. Select Email Address in the drop down for phone number type 
  5. Go to Online Express
  6. Click Account>Integration Options
  7. Select email mapping from the left hand menu
  8. Ensure that the drop downs for email mapping include the email type that was missing before