When searching in the CRM, the search uses non-alphanumeric characters to separate search terms. If an alphanumeric string is longer than 20 characters it cannot be indexed, and therefore does not appear in searches.
It is not uncommon for email addresses to have an alphanumeric string of 20+ characters, which means it cannot be searched, and if the string is 123456789012345678901@gmail.com, the search will only find anything meeting the criteria of 'gmail' and 'com', as the other term is not indexed and therefore wont be found when searching for it.
We should add an explicit 'or email =searchterm' to the contact search at least so that this can be used for all emails, as searching by email is very common.