Implement good looking HTML emails which follow best practice and comply rules.
Offer a setting to use text version instead of html one. (default will be html from now on, the text version will remain the unchanged current version with some minor tweaks)
Apparently SMF does just sent raw text mails out, including urls.
Because we use tracking, the url will be simply changed which leads to a longer url in crypt form.
I already added a subdomain tracking.ugx-mods.com to get rid of the different TLD, but it's still long (and scary) for users.
A simply html link should fix the issue, where the text is the correct url and the link url will be then replaced by the our mail sender provider.