Adding the email field to a filter in the Contact Search Center makes it impossible to save that filter.
[STEPS TO REPLICATE]
1. Open the Contact Search Center
2. Search By 'E-Mail'
3. Click the Save button
The options to 'Save as Filter' and Save as Group' are shown inactive
Filters can only but built using Contact1 and Contact2 fields, as the E-mail address is stored in the Contsupp table you are unable to build a filter using this field therefore the option to 'Save as filter' and 'Save as group' is disabled in the contact search center when you search on E-mail.
Other 'search by' fields that also have this restriction are:
Contact (Primary or additional)