QuickerSite is available on GitHub as from 2020.
You may want to star, fork, follow, download and/or contribute to the project over there. Thank you!
Yahoo has adopted a new security policy. Search Google for "yahoo dmarc changes".
This has an important consequence in case you use a Yahoo-email as the sending address for email messages. It simply does no longer work (in most cases). It depends on the SMTP server settings though.
The workaround would be to use custom SMTP server settings in your QuickerSite admin panel (available from QuickerSite 4.0).