iPhone 8

If you can't send an email on your iPhone, iPad or iPod touch

1. When you make an iOS or iPadOS backup in iCloud or iTunes, it backs up your mail settings, but not your email. ... 
2. Make sure your device is connected to the Internet.
3. Check with your email service provider to find out if there's a service outage.

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If you can't send an email on your iPhone, iPad or iPod touch ...

It would be better if you can please re-post with a screenshot, in order to understand the phenomenon better. Images/pictures speak louder than words. 

Please understand this is a public forum: While attaching a Screenshot, do not provide any personal credentials, such as IP address, Card detail, email id, apple id, IMEI number, Serial Number, Phone number, Order ID, if you are sharing a map ensure your location is not identifiable, etc.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.