Help me understand whether this is a gnutls.el bug

[João Pedro]

Hello folks,

Since a couple of weeks ago I started getting, from time to time, the
following message on my echo area

gnutls.el: (err=[-15] An unexpected TLS packet was received.) boot: (:priority 
NORMAL:%DUMBFW :hostname smtp.gmail.com :loglevel 0 :min-prime-bits nil 
:trustfiles (/etc/ssl/certs/ca-certificates.crt) :crlfiles nil :keylist nil 
:verify-flags nil :verify-error nil :callbacks nil)

and since I started receiving that message, some e-mail I send with
`message-mode' and `smtpmail.el' are not getting delivered (which is
strange given that if I open the Gmail web app I see them on the Sent
folder, but I can confirm that sometimes they're not delivered).

Does this sound like a bug with Emacs? Or is it something that changed
on Gmail's side? I'm using msmtp (and it is symlinked to `sendmail' in
order for `smtpmail.el' to use it) and `notmuch' on top of
`message-mode'.

Cheers (and I hope this one gets through...)!

-- 
João Pedro de A. Paula
IT bachelors at Universidade Federal do Rio Grande do Norte (UFRN)