Can't send mail using Exchange Online High Volume SMTP Server

Question

Pretty easy to replicate.

On the Adaxes server, you can use Powershell to send to that SMTP server so we know it's not a firewall issue:

$myCredential = Get-Credential -Username "it-noreply@contoso.com" -Message "Enter your password"
Send-MailMessage -From "it-noreply@contoso.com" -To "ngb@contoso.com" -Subject "test" -body "test" -SmtpServer smtp-hve.office365.com -UseSsl -Port 587 -Credential $myCredential

I've got a packet capture of both the Adaxes attempt and the Powershell success that I can send to you in an email. The packet capture shows the Adaxes server doesn't issue either an EHLO or a STARTTLS command.

Answer 1

Hello,

This behavior is expected. The thing is that you are using Microsoft 365 as SMTP server and it does not support SSL. We cannot comment why the Send-MailMessage works. Most probably, it is using an old library which had a bug making SSL be ignored.

To remedy the issue, clear the The server requires an encrypted connection (SSL) in the Adaxes mail settings. It does not mean your mail will be sent unencrypted. If you specified a secure port (465 or 587), Adaxes will use TLS, which is effectively an upgraded version of SSL with fixed security vulnerabilities.

Comments

Thanks - that did the trick!