I know this isn’t a solution, but I found a workaround to your issue.
I am having the same problem, with what it sounds like a similar setup to you. Emails sent to [mobile number]@vtext.com from the gmail information I have in the MX1EP don’t seem to go through from my Elk system. Even directly sending an email directly from the Gmail account to the vtext.com email address don’t go through either. So it must be an issue, like you said, between Gmail and Verizon.
How I ended up getting something to work is changing all the “[mobile number]@vtext.com” in the 2nd column for email messages 1-16, to [my gmail username]@gmail.com. Basically, putting the same information in the from, user name, and email address fields in the email tab of the MX1EP Setup box. What you end up doing it sending an email to yourself. Now, I have a dedicated Gmail address for my Elk, so the only thing that uses that Gmail address is the MX1EP, and depending on your phone, you may be able to make a unique notification sound for incoming emails from that Gmail account only (my Samsung allowed me to do that). So if you don’t have a dedicated email address for that, you might want to set yourself up one to isolate it out. The only disadvantage I can see (again, may be depending on your phone’s settings) is that preview on my phone only shows I received an email and just shows the subject “Alert from M1”. I have to open the email to see the message the panel is sending me.
Hope that helps and if you have any questions, let me know!