In April 2026, Microsoft made changes to the AirSync protocol. Because of this, some free and personal Outlook accounts can no longer send messages in eM Client 9. This version uses AirSync protocol for sending by default, so you may see the following errors when trying to send emails:

• VersionNotSupported
• The input string '' was not in a correct format.
• Invalid namespace Meeting Response

If you want to keep using version 9, you can switch the sending protocol from AirSync to SMTP.

On Windows:

Close eM Client. Download and run this configuration file to update the Windows registry.

You can also change it manually if you prefer:

• Run REGEDIT in Windows Start menu
• Navigate to HKEY_CURRENT_USER\Software\eM Client
• Right-click > New > DWORD and name it UseAirSyncSendMail

On macOS:

Close eM Client. Open Terminal and run this command:

defaults write com.emclient.mail.client UseAirSyncSendMail -bool FALSE

After switching to SMTP, open Menu > Accounts (or eM Client > Accounts on macOS) and disable AirSync under Services. This stops the errors, but Outlook calendar and contacts will no longer work. You can use the local calendar and contacts or add these services from another provider.