Update 10th July 2025: eM Client in versions 9.2.1628 - 10.3.2412 used AirSync instead of SMTP to send emails for free Microsoft email accounts - these are any accounts with domains @outlook.com, @hotmail, @live, @msn and other domains which Microsoft offered in the past, with all regional variants as well.
This change was made a few years ago when Microsoft broke SMTP Authentication for 3rd-party apps, making it impossible for us and many other apps to send emails.
You can read about this on our blog: https://www.emclient.com/blog/workaround-for-the-sending-issue-via-smtp-for-microsoft-outlook-com-accounts-570
Versions newer than 10.3.2545.0 now use SMTP again so the workaround below is no longer needed.

While AirSync does not suffer from the authentication issue at this time, Microsoft does not let it use ALL sending features - namely, any alias will revert to your primary email and send the email from primary address instead of your desired alias.
This, of course, does not affect users who don't need alias for these specific accounts, but if you need the alias feature to work correctly, you'll have to manually switch to SMTP.

Follow the steps below:

Windows

• Close eM Client.
• Open Start menu and type in REGEDIT and press Enter to open Windows Registry
• Go to Computer\HKEY_CURRENT_USER\Software\eM Client
• Right-click inside this section and create New > DWORD
• Name it UseAirSyncSendMail and save - this should set it to 0

macOS

• Close eM Client.
• Open Terminal and run this command: defaults write com.emclient.mail.client UseAirSyncSendMail -bool FALSE