Some email services, including iCloud mail accounts, require authentication for their SMTP server. If you verify that all your account settings are correct, but you still can't send messages, try turning on SMTP authentication.
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On the Tools menu, select Accounts.
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In the left pane of the Accounts dialog box, select the account.
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Under Outgoing server, select More Options…
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On the Authentication pop-up menu, select the authentication type, and then enter your credentials as necessary.
