Sometimes when you are trying to send mail through .Net code you will face following error.
5.7.0 Must issue a STARTTLS command first. pu4sm8968093pbb.72
I found 2 main reasons for this.
Generally SMTP server used 25 port but when you are going with secure server TLS settings then for secure server your Port should be 586
Enable SSL settings:- the second most important reason of showing this error is SSL settings when you sending mail through code. If you are using secure server or TLS settings for mail sending then Enable SSL setting for mail client should be true.
I hope this will resolve your problem too.
Thanks & Best Regards,