Sending, receiving, and organizing email in Outlook.com
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I'm sorry to hear that you're having trouble sending emails from your aliases in Outlook on the web. This may happen if your email is not configured to send email using the alias as the default email address. To fix this, you need to enable the Send from alias feature in your Outlook settings. Here are the steps you can follow:
• Sign in to your Outlook on the web account using your primary email address and password.
• At the top of the page, select Settings and choose Options.
• In the Options pane, under Mail, go to Accounts and select Connected accounts.
• Under From address, select Change your From address.
• Choose the alias you want to use as your default from the drop-down list and select Save
This should allow you to send emails from your alias without any issues. You can also check this support article for more details and screenshots: