Unable to send from outlook classic to multiple contacts

Hello Cedric,

Thank you for your question.

An undeliverable message does not necessarily mean there is a problem with Outlook itself. The failure could be related to the recipient addresses, your email account, or the receiving mail server.

Let's first check whether the problem occurs when sending to individual recipients or only when sending to several contacts.

1. Test sending to individual recipients

In Classic Outlook, create a new email and manually enter the complete email address of one contact. Avoid selecting a suggested AutoComplete entry for this test.

If the message is delivered, try sending to two or three contacts in the same message.

When adding multiple recipients, make sure their addresses are separated by semicolons (;).

Microsoft explains how to add recipients here:

Create an email message in Outlook

2. Check the Non-Delivery Report (NDR)

When Outlook displays the undeliverable notification, open it and look for the SMTP error code and the section labelled Diagnostic information for administrators.

The exact error is important because different codes indicate different problems. For example, 5.1.1 generally indicates that a recipient address could not be found, while 5.7.1 can indicate a mail policy or security restriction.

Could you confirm the following?

Which email account are you using in Outlook 2024: Microsoft 365, Outlook.com, Gmail, or an account from another provider?

Can you send successfully to one contact, or do you receive an undeliverable message even when sending individually?

What is the exact SMTP error code and message shown in the NDR?

Please remove any personal email addresses before sharing the error details.

With the NDR information, it will be possible to narrow down whether the issue is related to Outlook, the sending account, the recipient addresses, or the receiving mail server.

Best regards,

It can be one of many issues with the outlook auto-complete recipients, incorrect contact details or the outgoing mail server. Without seeing the Non Delivery Report (NDR), it is hard to tell exactly what the issue is.

To help troubleshoot:

Try sending just one recipient:

Create a new email and manually type the full email for just one recipient.

Remove incorrect auto-complete:

If the message went through after typing the email manually, try removing the incorrect auto-complete. Create a new message and type the email address for the recipient. Highlight the incorrect auto-complete and hit delete.

Outlook Web:

Log into your email providers webmail and try sending a test email. This will help to see if it is a specific issue with Classic Outlook.

Check recipient list:

If you are able to send to individual recipients but not multiple contacts, ensure the email addresses are correct. Try sending to fewer recipients. In Classic Outlook, separate the emails with semi-colons.

View the NDR:

Open up the undeliverable message. Look for the SMTP error code. Look at the section 'Diagnostic information for administrators'. This will show if the issue is with outlook, the outgoing server or the recipients mail server.

Could you let us know:

Are you able to send to just one contact or are all messages failing?

Are you using Gmail, Microsoft 365, Outlook.com or another email provider?

What is the exact error code or message shown on the undeliverable?

The SMTP error code is very important as different errors will need different solutions. For example, an invalid recipient error (5.1.1) would have a different solution than a security or policy issue (5.7.1).

I will resend my question