Mail

Check your iCloud storage Check your iCloud storage online or on any device - Apple Support

New mails won't arrive when iCloud storage is exceeding , if so manage iCloud storage Manage your iCloud storage - Apple Support

See the article If you can't receive email on your iPhone, iPad, or iPod touch - Apple Support

Dydymond Said:

"Mail: I haven’t been receiving emails on the mail app for a while i’ve been missing important emails and much more , how can i fix this ?"

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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