My most useful config in ~/.muttrc
Editor
Always in vim insert mode in both send and reply mode
set editor="sh -c 'grep -q \"^On \" \"$1\" && vim -c \"normal ggO\" -c \"startinsert\" \"$1\" || vim -c \"startinsert\" \"$1\"' _"
Signature
set signature = "echo 'An Vo\nSaigon | anvtl.com\n'|"