When using certain mail servers, it is necessary to use the Punycode representation of the domain name instead of the UTF-8 representation. For example, "foo@mañana.com" is technically "firstname.lastname@example.org" and needs to be written as such for non-Punycode-aware mail servers. However, XWiki removes the double hyphen and replaces it with strikethrough formatting, which is confusing. A similar problem occurs if the email address happens to contain a double underscore.
1. Go to http://localhost:8080/xwiki/bin/view/Invitation/
2. Send an invitation to email@example.com
3. Observe the result.
4. Click "My Invitations" and observe that this page has the problem too.
Screenshots of step 3 and step 4 are attached.