Unable to receive emails with MailEnable: SSL recv failed

Symptoms

• Some subscriptions are not receiving emails from Google, Outlook, etc.
• Following errors are found in C:Program Files (x86)Mail EnableLoggingSMTPSMTP-Debug-<date>.txt MailEnable log file:

  ...
  ME-I0148: [0F95DD61854F47A5A79AE4974C40BB02.MAI] DATE (DATA Termination) command failed with temporary return code.
  ME-E0034: [0F95DD61854F47A5A79AE4974C40BB02.MAI] DATA command returned with a 421 response, meaning that the entire message should be retried.
  SSL recv failed: socket has was closed gracefully by peer before this recv [1412]
  ...
  ME-I0070: [CC0FF2408468408D8C46F4915CFF2824.MAI] (recv) socket [1956] was gracefully closed during [RCPT] command by the remote client 203.0.113.3.
  ME-I0074: [1956] (Debug) End of conversation
  ...
  ME-I0070: (recv) socket [1036] was gracefully closed during [AUTH] command by the remote client 203.0.113.2.
  SSL recv failed: socket has was closed gracefully by peer before this recv [1956]
  ...

• Invalid SSL certificate is used to secure Mail server connections in Plesk > Tools & Settings > SSL/TLS Certificates:

  

• Checking with openssl utility provides following errors:

  # openssl s_client -connect plesk.example.com:25 -starttls smtp
  CONNECTED(00000003)
  didn’t found starttls in server response, try anyway…
  # openssl s_client -showcerts -connect plesk.example.com:465
  socket: Connection refused
  connect:errno=111

Cause

Invalid certificates and DNS servers are configured in MailEnable.

Resolution

1. Login into Plesk GUI
2. Secure Mail server with a valid certificate: How to secure mail server with Let’s Encrypt certificate?
3. Connect to the server via RDP
4. Ensure valid DNS servers are set in MailEnable > Servers > localhost > Services and Connectors > Right click on SMTP > Properties > Change DNS servers, use a realiable DNS servers such as Google DNS servers with IP 8.8.8.8 and 8.8.4.4 > Apply changes > Right click on SMTP > Stop > Right click on SMTP > Start