Update: this is fixed in Mailpress version > 5.3
Using the MailPress tracking add-on changes all the links in the email to special tracking links. When you are using anchor links in your email newsletter these links are also changed to tracking links and will not work anymore. There is a solution all though hacking the core files is never a good option. I could not find an alternative so here it is:
in mp-content/add-ons/MailPress_tracking.php near LINE: 127
under the line:
if (strpos($match, 'mailto:') !== false) continue;
if (strpos($match, '#') !== false) continue;
This will leave all ‘anchor-links’ untouched.