Trying to Send SMTP via an HVE account comes up with the following error :

Send-MailMessage : The SMTP server requires a secure connection or the client was not
authenticated. The server response was: 5.7.57 Client not authenticated to send mail. Error: 535
5.7.139 Authentication unsuccessful. Contact your administrator.
At C:\Scripts\SMTP.ps1:10 char:1

• Send-MailMessage -From [email protected] -To [email protected] …
• ~~~~~~~~~~~~~~~~~
  • CategoryInfo : InvalidOperation: (System.Net.Mail.SmtpClient:SmtpClient) [Send-Mai
    lMessage], SmtpException
  • FullyQualifiedErrorId : SmtpException,Microsoft.PowerShell.Commands.SendMailMessage

Make sure you whitelist the account from Any Conditonal Access Policies that block basic Auth or MFA

If your MFA policy allows via Trusted IPs then you are good

Create an Authentication Policy that allows Basic SMTP

 Get-AuthenticationPolicy | Select Identity 

Get-OrganizationConfig | Select DefaultAuthenticationPolicy 

New-AuthenticationPolicy BasicAllowSMTP -AllowBasicAuthSmtp

 Set-User -Identity "[email protected]" -AuthenticationPolicy "BasicAllowSMTP"