Why does Send-MailMessage fail to send using START

Question

Why does sending an email message from PowerShell with the Send-MailMessage command using the flag -Port 587 produce an error.

Command:

Send-Mailmessage -smtpServer mail.server.com -Port 587 -from "[e-mail]" -to "[e-mail]" -subject "Test" -body "Test"

Error Message:

Send-Mailmessage : The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Must issue a STARTTLS command first

The PowerShell documentation says adding -UseSSL should specify that a STARTTLS command be sent, but even adding this flag may not resolve your issues.

Command:

Send-Mailmessage -smtpServer mail.server.com -Port 587 -UseSsl -from "[e-mail]" -to "[e-mail]" -subject "Test" -body "Test"

Eror message:

Send-Mailmessage : Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.

Answer 1

Some SMTP servers may have been hardened to only accept TLS 1.2 for negotiating STARTTLS. In many cases Windows is configured to send TLS 1.0 by default when -UseSSL is specified.

To force Send-MailMessage to use TLS 1.2 it is necessary to add a line to the script before executing the Send-MailMessage:

Either enter:

[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12

or

[System.Net.ServicePointManager]::SecurityProtocol = 'TLS12'