spectrum imap email setup for outlook 2003

I would not attempt a change unless I had done some testing and verified that a different email service would fix the problems. MS365 would be an expensive solution for multiple email addresses. I have several clients using Ionos email with custom domains, and changing them to a different service would be possible but challenging. I have tried talking to Ionos customer service in the past, but that was before I realized the apparent cause of the problem.

Nothing I've tried seems to be fixing this. I'm using only TLS1.2 (set in Internet Options). I've verified email account settings, server settings, etc.

If I delete the name and email address from the autocomplete, it still eventually finds the name in the address book, fills in the person's name, but the email address is not included, so I don't think it's a corrupted autocomplete entry. I've checked the messages on the Ionos webmail, and the email in the sent items folder says 'No recipients' in the To column. My Spectrum IMAP email doesn't seem to be bothered by this, but the Ionos accounts don't like it at all. Outlook autocompletes using just the person's name.

This seems to happen if the contact/global address book listing (from an Exchange or other IMAP account) has a person's name and email address. Further digging shows that, in the message header, the To: line has just the person's name in quotes (like 'John Doe') with no email address after it. When I try to send IMAP messages on my Ionos account (I have more than one), sometimes the server says the messages cannot be delivered. They're all entered as accounts in Outlook 365. I have some Exchange email addresses and some IMAP email addresses from different providers. I have an interesting problem with Outlook 365 (business license).

Spectrum imap email setup for outlook 2003