did you do the mapping from the field of moodle and your external database
if not go to there
Data mapping (Email address) auth_db | field_map_email
And play with these three field if neccessary
Update local (Email address) auth_db | field_updatelocal_email
Update external (Email address) auth_db | field_updateremote_email
Lock value (Email address) auth_db | field_lock_email
But with what you said the most logical problem is that they've a mapping problem with email field of the external db and the Moodle one.
Hope it's help.