Yes, you can import only the email address. Import-O-Matic automatically uses Email Address as one of its duplicate criteria, provided that you tell it to check for duplicates. Since there is no other criteria to Auto-Pick on Import-O-Matic will display the Duplicate Search Matching Results screen for each row. But, it will match them. The issue is if the emails being imported are different that what is on the constituent record, it will not be able to match to the constituent. What is the desired outcome for this import?
If you do decide to import the email address only, I would suggest setting your criteria to:
ID Matching Options: Set to Continue processing the data row Duplicate Searching Options: Set to Perform a duplicate search when a record cannot be matched by ID and Search for non-constituent spouse record match
Please let us know if you have any additional questions.