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The url created by the download page is incorrect for PHP based web servers when an email address contains a plus. For example, bob+smith@gmail.com (mailto:bob+smith@gmail.com). PHP magically converts it to a space unless special measures are taken. The workaround is to replace the plus character with %2b as in bob%2bsmith@gmail.com (mailto:2bsmith@gmail.com).
You may wonder if a plus character is a legitimate character in an email. It is, please consult the email RFC 5322 section 3.2.3:
https://tools.ietf.org/search/rfc5322 (https://tools.ietf.org/search/rfc5322)