I can not send out emails from my phone

from my mobile app

Do you use a special app or do you mean iOS Mail app?

Try first to optimize the range of the mobile network, see this Apple article:

If you see No Service or Searching on your iPhone or iPad

If you see No Service or Searching on your iPhone or iPad - Apple Support

Check the Wi-Fi network, see the following Apple articles:

Connect to Wi-Fi on your iPhone, iPad, or iPod touch

Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support

If they are On, turn Off Hotspot & VPN.

Then read & follow the steps in -> Related Article -> If you can’t send email on your iPhone, iPad, or iPod touch

NScorpio Said:

"I can not send out emails from my phone: I can’t send emails from my mobile app"

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Use a New App Password:

Do you use an App Password? If so, try a new App Password. Sometimes, changes (i.e. updates) are made to the device, and then mail server syncing becomes an issue (which is what a new App Password it intended to fix). Go here for gmail: Sign in with App Passwords - Google Account Help - Goog