According to MS Outlook service, Exchange Online no longer supports use of TLS1.0 and TLS1.1 in the service as of October 2020. This change is due to security and compliance requirements for its service. While no longer supported, outlook servers still allow clients to use those older versions of TLS when connecting to the SMTP AUTH endpoint (smtp.office365.com). In 2022, they plan to completely disable those older TLS versions to secure customers and meet those security and compliance requirements. However, due to significant usage, they have created an opt in endpoint that legacy clients can use with TLS1.0 and TLS1.1. Note that this endpoint is not available in GCC, GCC-High, or DoD environments that have legacy TLS permanently turned off.
Possible Solutions and Work Around:
A. You change the end-point server to "smtp-legacy.office365.com" because the "smtp.office365.com" will no longer working.
MailMessage mail_message = new MailMessage();
mail_message.From = new MailAddress(k_business_email.ToString());
mail_message.To.Add(new MailAddress(p_email));
mail_message.Subject = p_subject;
mail_message.Body = p_body;
mail_message.IsBodyHtml = true;
//
SmtpClient smtp_client = new SmtpClient();
smtp_client.DeliveryMethod = SmtpDeliveryMethod.Network;
smtp_client.Host = "smtp-legacy.office365.com";
smtp_client.Port = 587;
smtp_client.UseDefaultCredentials = false;
smtp.Credentials = new System.Net.NetworkCredential("user_id", "password");
smtp_client.EnableSsl = true;
B. Change your email server on "Exchange Admin Center".
1. Login in your office account.
2. Go to admin page "https://admin.exchange.microsoft.com/". select "Settings", then select "Mail Flow",
3. Check the checkbox "Turn on use of legacy TLS client".
In this way, it will allow you to continue using the previous version of SMTP email service.