Outlook for Mac Error 17099: Email Sending Troubleshooting Guide

What Error 17099 means

Error 17099 is an Outlook for Mac send-failure code. When Outlook cannot transmit a message through the outgoing mail server (SMTP), it generates this error and typically holds the message in the Outbox.

The underlying cause can be authentication failure (wrong password, expired credentials, app-specific password required), incorrect SMTP server settings, a network or firewall blocking the connection, or a server-side policy change — for example, your organisation enabling multi-factor authentication.

Check account settings

1. 1

   Open Outlook Preferences > Accounts

   Check the outgoing mail (SMTP) settings for the affected account: server address, port and encryption method.

2. 2

   Verify authentication credentials

   Outlook may be storing outdated credentials. Remove and re-enter the password, or use 'Update Password' if the option is available.

3. 3

   Check for app-specific password requirements

   If your email provider requires app-specific passwords for third-party clients (some Google and Microsoft configurations do when advanced security is enabled), an app-specific password may be needed.

Remove and re-add the account

Removing the account and adding it fresh causes Outlook to re-fetch current settings from the provider's auto-discovery service, which often corrects outdated configuration.

1. 1

   In Outlook, open Preferences > Accounts

   Select the affected account.

2. 2

   Remove the account

   Click the minus button. Outlook will warn you that local data may be removed.

3. 3

   Re-add the account

   Click the plus button and use the automatic setup option with your email address. For Microsoft 365 and Exchange accounts, automatic discovery should populate settings.

Network and firewall checks

Some workplace networks block outbound SMTP connections on standard ports. If you are on a VPN or a managed corporate network, your IT team may need to confirm that Outlook's outgoing connections are permitted.

Test email sending on a different network (mobile hotspot) to determine whether the error is network-specific.

Update Outlook

Microsoft regularly updates Outlook for Mac with bug fixes and server-compatibility improvements. Check for updates via Help > Check for Updates or through the Mac App Store if you installed Outlook that way. Running an old version can cause authentication and sending failures as server requirements change.