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I am trying to build an application that needs to send out a lot of emails with following requirements.

  1. The application should be able to send emails on daily basis(to be specific, 20-30 mails on average but at times the number may reach 60 at max). I cannot add all recipients to BCC and send a single mail as the message body of each recipient is different.
  2. The emails need to be secure, i.e., network security.

When testing using Google's SMTP server, I tried testing with 30 mails by enabling TLS and everything seems to be working fine. I did the testing using the following code.

SmtpClient client = new SmtpClient("smtp.gmail.com", 587);
client.EnableSsl = true;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
client.Credentials = new NetworkCredential("username", "password");
client.Send(msg);

However I have a few queries.

  1. The email sending limit of Google's SMTP server is 100. I am using port 587. In another article it is mentioned to use port 465. Is there any significance of the port. If so, then which port should I use if I have to keep the daily sending limit in mind?
  2. I also need to ensure that the email sent is secure and no 3rd party is able to eavesdrop. Is SMTP over TLS enough?
  3. What are the pros/cons of using an ESP like Mandrill? Do they ensure network security?
SohamC
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    Port 587 uses TLS security and port 465 uses SSL security. Both are secure mail protocols except some basic differences. .Net framework's managed code supports using TLS security (which you are using). For using SSL security you need to use obsolete classes. Still you can achieve that. [Here's how](http://stackoverflow.com/questions/37070767/send-e-mail-using-ssl) – Kamalesh Wankhede Aug 08 '16 at 06:44

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