Publons collects information about peer reviews and builds public reviewer profiles for participating reviewers. To do this we record information regarding individual reviews using the journal name and the date the review was completed

A reviewer certificate, acknowledging a reviewer for their overall contribution to the journal does not show which reviews have been performed or on which dates they were completed, thus we cannot process a review certificate as a review receipt to verify and add an individual review.

For the same reasons we cannot process general thank you for reviewing emails. General thank you emails are emails which are not specific to any manuscripts and are thanking the reviewer for the process and not the completion of a review.  

e.g. "We would like to thank you for all your efforts and contributions to our journal for the year."