When users attempt to delete a primary email address they may receive the error "A primary email address cannot be deleted".
Typically in this scenario, the constituent has an inactive email address on their record. This can be seen by selecting the filter show former contact information. The application does not allow you to delete a primary email address leaving another one on the record that is primary.
To resolve this issue, you will need to add another email and mark it as primary or remove the end date from the former email and mark that one primary. Once this is complete you can then delete the email address in questions.
Steps to Duplicate
1) Navigate to a constituent record 2) Select the Contact tab 3) Under Email Address attempt to delete the primary email address.