Hey there New_to_iphone11,
Thanks for reaching out to Apple Support Communities. We understand your emails are missing unexpectedly. We’ll be happy to help with this.
To verify, does this happen for multiple accounts or just certain ones? Log into your affected email accounts from a web browser and see if your missing mail shows on the account level. This can help us determine what direction we need to head to resolve this.
If it shows on your account but not on your iPhone, back up the emails before taking any other steps. Then use the search option in Mail and see if your missing messages show in the results:
Search for and view email on your iPhone, iPad, or iPod touch
If they don’t show, go to Settings > Mail, then tap Accounts. Tap on the affected account and then tap Delete Account. Restart your iPhone after this:
Restart your iPhone
Add the account back and see if the missing emails show:
Add an email account to your iPhone, iPad, or iPod touch
We hope this helps.
Take care.