Outlook Error Codes

Fix Outlook Error 550 When Sending Yahoo Calendar Invitations

Rana Garcia Oct 9, 2026 869 views

Question details

The user needs to resolve an Outlook error (550 Request failed; Mailbox unavailable) that prevents sending Yahoo calendar invitations, despite regular emails sending without issue.

Fix Outlook Error 550 When Sending Yahoo Calendar Invitations

Product: Microsoft Outlook
Device & OS: not provided
Scenario: Sending calendar invitations using a Yahoo account configured in the Outlook desktop application.
Observed behavior: Regular emails are sent successfully, but calendar invitations fail and return a '550 Request failed; Mailbox unavailable' bounce-back error.

Before you start

Log into your Yahoo Mail account via a web browser and try sending a calendar invitation directly from the web interface to verify if the issue is strictly isolated to the Outlook desktop client.

Solution 1Recommended

Switch to the Active Local Calendar

Use this solution if you have multiple local calendars in Outlook. The error often occurs when Outlook attempts to send invitations from an older, superseded local calendar instead of the active one.

When configuring IMAP accounts like Yahoo in Outlook, the application creates local calendars labeled 'This computer only'. Over time or after updates, duplicate calendars such as 'This computer only (2)' can be generated. Sending an invite from the older, inactive calendar will trigger the 550 error.

Open the Calendar View

Launch Microsoft Outlook and click on the Calendar icon located at the bottom left or in the left navigation pane.

Locate Duplicate Calendars

Look at the left panel under 'My Calendars'. Check if you have multiple entries labeled 'Calendar (This computer only)' and 'Calendar (This computer only) (2)'.

Select the Newer Calendar

Uncheck the older calendar and check the box next to the newer calendar (usually the one with a number in parentheses) to set it as your active viewing calendar.

Create a New Invitation

Click 'New Meeting' or 'New Appointment' while the newer calendar is selected, and send the invitation to test if the 550 error is resolved.

SMTP Settings Note: Do not alter your Yahoo SMTP settings. There is no separate SMTP configuration specifically for calendar invitations; they use the same settings that successfully send your regular emails.

Solution 2

Create a New Outlook Profile

If changing the calendar does not work, creating a fresh Outlook profile can resolve deep-rooted synchronization and configuration errors with your Yahoo account.

Solution 3

Repair the Microsoft Office Installation

Use the built-in repair tool to fix corrupted Microsoft Office application files that might be interfering with Outlook's calendar functions.

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Frequently Asked Questions

Why do I get a 550 Mailbox Unavailable error only on calendar invites?

This error typically triggers when Outlook attempts to send a meeting invitation from a deprecated or corrupted local 'This computer only' calendar, rather than your primary active IMAP calendar. It is a sync and calendar selection issue, not a general email blocking issue.

Are there different SMTP settings for Outlook calendars?

No, Outlook does not use a separate SMTP configuration for calendar invitations. If your regular emails from your Yahoo account are sending successfully, your SMTP settings are perfectly fine.

How can I tell if my Outlook profile is corrupted?

Signs of a corrupted Outlook profile include erratic synchronization behaviors, such as emails sending fine but calendar events failing, persistent error codes (like 550 or 0x800CCC0F) despite correct server settings, or the application repeatedly crashing upon launch.

Microsoft Office Error Codes