How can I recover lost emails after iOS 18.5 update?

The best part of IMAP is it does not remove the emails from the server unless you delete them. See the pic below for Gmail & Outlook

If you added IMAP email accounts, the iOS mail app does add IMAP by default.

When you set up an email address in an email client, you will need to decide if you want to set it up as POP3 or IMAP. Both of them are ways to connect to the mail server so you can read your emails through an email client.

• IMAP is short for Internet Message Access Protocol. With IMAP, the message does not remain on the local device, such as a computer, it remains on the server.
• POP3 is short for Post Office Protocol. With POP3 mail, it will connect and attempt to keep the mail located on the local device (computer or mobile).

Pop3 vs. IMAP: Which is better?

• IMAP is better if you are going to be accessing your email from multiple devices, such as a work computer and a smart phone.
• POP3 works better if you are only using one device, but have a very large number of emails. It is also better if you have a poor internet connection and need to access your emails offline. For most people, IMAP will suit their needs better.

Note: If emails start disappearing from your inbox without you deleting them, it almost always means one of your devices is on POP3. Backing up the emails and then resetting up the email account as IMAP will fix the problem.

It's not always the iOS update's fault!  Many things can cause phone problems.  If you tell people you think it's an iOS update, it might make it harder to fix or find a solution for the real issue.

Remember, the right question often leads to the solution!

Was/Is your device Jailbroken? If that's the situation, please be informed that attributing the issues to iOS updates may not be accurate.

iOS updates aim to enhance user experience, boost security, and fix bugs rather than disrupt core features. The majority of the issues after updates might stem from underlying, unmentioned factors. Updates won't typically alter personal data but may refine settings or features. Backup before major updates for safety. Review Apple's update details and backup advice for a smooth process.

Kindly consider checking your settings and addressing any device issues before attributing the issue to external factors. This could be a pure coincidence that the iOS update affected the iPhone functions.

Before initiating a standard iOS update, it's advisable to create a backup. Have you completed this step? If yes, proceed with the restoration from the backup.

See what each update does: About iOS 18 Updates, About iOS 17 Updates, About iOS 16 Updates, About iOS 15 Updates, About iOS 14 Updates, About iOS 13 Updates, About iOS 12 Updates, and so on...