Client SMTP submission using Basic authentication in Exchange Online is scheduled for deprecation in September 2025. We strongly recommend using one of the following alternative methods instead:
Send email to internal recipients only: Use High Volume Email for Microsoft 365. For instructions, see Manage high volume email for Microsoft 365 Public preview.
Send email to internal and external recipients: Use Azure Communication Services Email. For more information, see Email SMTP support in Azure Communication Services.
If you have one or more on-premises email servers (Exchange Server or any other SMTP server), you can use one of the following methods:
Use Basic authentication to authenticate with the on-premises email server.
Configure the on-premises email server for anonymous relay (not open relay). For instructions in Exchange, see Allow anonymous relay on Exchange servers.
Client SMTP submission using Basic authentication isn't compatible with Security defaults in Microsoft Entra ID. We recommend using Modern authentication (OAuth) to connect to our service. For more information about OAuth, see Authenticate an IMAP, POP, or SMTP connection using OAuth.