Hi Silvan,
The following works with vim. It requires fzf and fzf.vim.
Once in a while, you can use
maddr \| sort \| uniq > ~/my.contacts
to dump all contacts found in your emails in that file.
Then in .vimrc:
inoremap <expr> <c-x><c-k> fzf#vim#complete('cat ~/my.contacts')
will let you complete emails inside vim using fzf when writing an email in vim.
Best wishes,
Pierre