Mail won't remember email addresses for autocomplete

Hello Chanticleer71,

Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with mail not autocompleting email addresses. Are htese addresses in your Contacts app? This could explain why this is occurring: Save and find email addresses in Mail on iCloud.com

"Save email addresses to your iCloud contacts

In Mail on iCloud.com, do either of the following:

• Add a contact from an email you received: View the message. At the top of the message, click the sender’s name or email address. In the window that appears, edit the contact details if necessary, then click Done.
• Add a contact while composing or responding to an email: Select an address in the To, Cc, or Bcc field. Click  next to the address, add or update the contact details, then click Done.

If your account has Screen Time communication limits turned on, you may not be able to add or edit contacts. See If you can’t create, import, edit, or delete contacts on iCloud.com.

Use addresses from your contacts stored in iCloud

1. In Mail on iCloud.com, open a new message.
2. Start typing the name or email address of a contact. As you type, matching addresses are displayed. Select one."

If the contacts were assigned to an email account, you would need to add that email account to your new iPhone to get your contacts.

We hope this helps.

Cheers!