Sent emails disappearing on iPhone

Connecting to Wifi is the first part and then getting connected to the Internet is the second

1. If you connect to Wifi, doesn't necessarily mean you are connected to the internet. If you are not connected to wifi could be a problem with iPhone settings or Router/Modem Settings
2. If you are not connected to the internet is the sole issue of Router / Modem Settings. Contact your ISP immediately

Check your WiFi Router / Modem for --> Recommended settings for Wi-Fi routers and access points ...

For your Internet connection.

• Over Cellular: contact your Carrier (Cellular Service Provider). The agency who issued the SIM to you
• Over WiFI: Contact your ISP (Internet Service Provider), the agency who provided the Internet Service on your Modem/Router to you

However --> Connect iPhone to the Internet - Apple Support

We also have had the same problem for nearly a year now. 2 x iPhones 12, iOS 16.3.1, service provider BT. The problem is intermittent and occurs when using the home router or when roaming, with both phones. We write an email, click send and the email just disappears, but sometimes the recipient receives it, sometimes they don't and then there's no copy in the 'sent' email folder. So if we don't check, there's no way of knowing whether the email has actually been sent. As a workaround we can save the email first before sending, that seems ok. Or we can select and copy the content of the email and paste it into another one, fingers crossed, that should go. Very frustrating, looked into various options to solve this but it's still occurring.