I can see that XenForo's transactional emails like watched threads are responsive however the account activation email isn't responsive because the large URL contining the activation link is being displayed in a single line.
IIRC if I receive an email about a watched thread and the title is long it wraps fine however the problem seems to be only with activation emails (same email client).
Not wrapping on the "/" seems like a failure of the mail program as this is a standard character to consider "breaking". Regardless, I have tweaked this.