I just fixed an issue with my setup for this same issue.
vtext (and mms.cricketwireless.net too btw.) apparently block messages without a valid MX record, or some similar type of sanity/security check.
Basically, my smtp server and my computer aren't the same and whatever vtext.com uses was aware of that and rejected this kind of behavior.
editing/adding the line
myorigin = smtp.servername.suffix
in main.cf for postfix (for me was /etc/postfix/main.cf your's might be different) fixed this issue for me.
My understanding is that this told the sms from e-mail server that the message was valid. i.e. where it says it is coming from is equal to where it is actually coming from.
Other than that, you can systematically check other aspects of your mutt configuration (i.e. mutt -D), postfix configurations, or try to tell mutt to use something like imap/smtp servers from elsewhere (like imaps://imap.google.com).
I don't think anything is going wrong with mutt, I presume vtext is a service where they convert your email into a text message? The issue lies likely with them. Can mutt send emails anywhere else? – djsmiley2k TMW – 2017-08-22T14:03:59.563
Mutt works locally, and emails from gmail are received as SMS messages on my phone. – Steve Mucci – 2017-08-22T20:12:03.560