PowerShell responds with the error “ The SMTP server requires a secure connection or the client was not authenticated.” In this case, setting up the secure connection is the problem.

An inbound connection for SMTP AUTH can be serviced by any Exchange Online server, but after Microsoft upgrades a server, it will reject TLS 1.0 and 1.1 connection requests and only accept email if the sending server allows messages to be passed in an unencrypted form. Exchange Online uses opportunistic TLS to negotiate connections with mail clients and other servers. The problem occurs because the workstation where the script runs attempts to negotiate a TLS 1.0 or 1.1 connection for the SMTP AUTH protocol. Our script to send a welcome message to new users is a good example of the type which will be affected. And when Exchange Online enforces TLS 1.2, it can stop PowerShell scripts sending email using the Send-MailMessage cmdlet via the SMTP AUTH protocol.

It will take time before every mailbox server declines old TLS connections, but it’s going to happen. Microsoft is in the process of removing TLS 1.0 and 1.1 support from Exchange Online to enforce the requirement to use TLS 1.2. Time to Review Use of Send-MailMessage Cmdlet with Exchange Online

Test tls 1.2 powershell