Weird. I looked over my config again, and don't see anything I omitted,
other than telling emacs/mu4e to use msmtpq in my .emacs.
--8<---------------cut here---------------start------------->8---
;; sending mail
(setq message-send-mail-function 'message-send-mail-with-sendmail
sendmail-program "/usr/local/bin/msmtpq"
user-full-name "Josiah Schwab")
--8<---------------cut here---------------end--------------->8---
Are you 100% sure that emacs/mu is using the new version of the script?
What happens if you test msmtpq, like you would msmtp?
https://wiki.archlinux.org/index.php/msmtp#Test_msmtp
That was how I debugged things when it was broken for me last year.
Josiah