MailJet progam (.net 5.0) & Azure VM's

Question

We have a custom .net 5.0 emailer program running in Azure VM's. We are using "2525" port and firewall rules looks OK. The main problem is, the emails are not being sent! Its throws the follow stack trace and it points to SMTP server issue. Is there anything else we can try or check from Azure VM's or inside the mail Jet application? Any suggestions are appreciated!!

Stack trace error - "System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.. ---> System.Net.Sockets.SocketException (10054): An existing connection was forcibly closed by the remote host. at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size) --- End of inner exception stack trace --- at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size) at System.Net.DelegatedStream.Read(Byte[] buffer, Int32 offset, Int32 count) at System.Net.BufferedReadStream.Read(Byte[] buffer, Int32 offset, Int32 count) at System.Net.Mail.SmtpReplyReaderFactory.ReadLines(SmtpReplyReader caller, Boolean oneLine) at System.Net.Mail.SmtpReplyReaderFactory.ReadLine(SmtpReplyReader caller) at System.Net.Mail.SmtpConnection.GetConnection(String host, Int32 port) at System.Net.Mail.SmtpTransport.GetConnection(String host, Int32 port) at System.Net.Mail.SmtpClient.GetConnection() at System.Net.Mail.SmtpClient.Send(MailMessage message) --- End of inner exception stack trace --- at System.Net.Mail.SmtpClient.Send(MailMessage message)"