How to configure SMTP Authentication with Outlook Express on a Mac
Outlook Express 5.02 has a bug in SMTP authentication that appears in some installations.
1. Open Outlook Express and go to the menu item Tools and choose Accounts.
2. Next, select the account you need to edit (i.e., an account that requires SMTP authentication). If you have multiple accounts that require authentication, you will need to edit them one at a time.
With the account name selected (highlighted), click on Edit. This will open the dialogue box shown below.
3. On the bottom of the page, you will see Click here for advanced sending options. Click on this link/button, and the additional dialog box Advance Sending Options will open. See below.
4. Now, put a check mark in the box for SMTP server requires authentication.
Then, put a check mark in the box for Log on using. See below.
5. Now, we’re getting to the Work Around part. The trick is to insert for the Account ID the username at domain_name, but instead of using the @ character, use the & character. See below.
Content retrieved from: https://support.appliedi.net/kb/a261/how-do-i-configure-smtp-authentication-on-a-mac.aspx.