Recently I found about sup  and gave it a try to replace mutt. One of my requirements is access to my Gmail contacts from within the client so I can auto-complete the To, CC and BCC fields with my already registered contacts.
Fortunately I already had a ruby script to integrate Gmail contacts with mutt  and since sup can be extended via ruby scripts it was extremely easy to get my script working with sup.
SUP email client with Gmail contacts
Sup hooks system  is very powerful and allows us to add all kinds of functionality to sup using simple ruby scripts. One such hooks is called "extra-contact-addresses.rb" and from the hooks documentation we can read:
What this means is that we must create an script inside our $HOME/.sup/hooks directory called extra-contact-addresses.rb that returns an array of email addresses in plain format (e.g. firstname.lastname@example.org) or full format (e.g. User Name <email@example.com>).
Here is the script I implemented for this purpose:
Simply copy this script to "~/.sup/hooks/extra-contact-addresses.rb" and your are ready to go. When composing or replying and email you can press Tab to autocomplete email addresses.