void-packages/srcpkgs/mailnag/files
..
README.voidlinux

Install gst-plugins-base1 if you want the Sound Notifications plugin.
Install libnotify and a libnotify client if you want the LibNotify Notifications plugin.
Install mailnag-goa-plugin if you want the GNOME Online Accounts plugin.
This package needs a libsecret client (GNOME Keyring, Kwallet...).
You should also have a working syslog (https://docs.voidlinux.org/config/services/logging.html) if you do not want error messages about missing syslog.