IMAP server rejects sent email - cannot deliver messages

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New Member
Outlook version
Outlook 365 64 bit
Email Account (as MS Exchange)
Hi all,
I have an interesting problem with Outlook 365 (business license). I have some Exchange email addresses and some IMAP email addresses from different providers. They're all entered as accounts in Outlook 365. 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. 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. 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. Outlook autocompletes using just the person's name. My Spectrum IMAP email doesn't seem to be bothered by this, but the Ionos accounts don't like it at all. I've checked the messages on the Ionos webmail, and the email in the sent items folder says "No recipients" in the To column.

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 verified email account settings, server settings, etc. I'm using only TLS1.2 (set in Internet Options). Nothing I've tried seems to be fixing this. I have tried talking to Ionos customer service in the past, but that was before I realized the apparent cause of the problem.

I have several clients using Ionos email with custom domains, and changing them to a different service would be possible but challenging. MS365 would be an expensive solution for multiple email addresses. I would not attempt a change unless I had done some testing and verified that a different email service would fix the problems.

I've posted this on the Microsoft Outlook forum as well, but thought I'd try here where there are very knowledgeable people.

Does anyone have any suggestions for things I might try?

John M
More info on this problem. I turned off the "Use Auto-Complete List to suggest names..." setting under Options > Mail > Send messages section, and now it is working differently. I can send messages from the Ionos account to an Exchange organization email address. It gets marked as spam/unverified user, but that is a problem for a different thread.
I will have to watch this for a while to see if it is a long-term fix or just an anomaly.

Now my question: is what part of the Auto-Complete is causing this behavior?
Yet more info: Ionos investigated the email that was rejected, and came up with the following analysis:

Your email was rejected by our SMTP server because it does not comply with the standards defined from RFC5321 and RFC5322.
Please make sure that the email client meets the following criteria for sending emails:
1. The header of your email must contain a valid "Date:" entry according to RFC2822: RFC 2822 - Internet Message Format

2. The following field types must be unique: "Date:", "From:", "Sender:", "To:", "CC:", "BCC:", "Subject:"

3. If the above field types are included, they must not be empty and have to be syntactically correct.
Note: The field types "To:", "CC:" and "BCC:" can, of course, contain several recipients.

We adhere to the recommendations of RFC7103 and do not automatically correct erroneous messages because this would invalidate DKIM signatures.

We apologize for any inconvenience this may have caused.

It's rather hard to believe that Outlook would not comply with the standards. My assumption is that they're sending out a template for any header error issues, and item 3 (syntactically correct) is what is causing this problem. I'm wondering if this was something that happened about 6 months ago, when I started having problems.

Anyway, since I can't change much in the code or configuration, until MS does something, I guess I'll keep Auto-Complete suggestions turned off. Sigh.
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