Opportunistic TLS is a protocol configuration that enables an email client to upgrade from a plain text connection that is not secure to an encrypted connection that is secure.
STARTTLS is the email protocol configuration that the email client uses to ask the email server for this upgraded connection. If this command fails, the email will be sent without encryption.
Forced TLS is a protocol configuration that ensures that before an email is sent, there is already an established TLS connection and therefore, messages are encrypted. Without this connection being established, the mail will not be sent to the recipient.
Secure/Multipurpose Internet Mail Extensions (S/MIME): These are other common secure alternatives to SMTP protocols used to secure emails with end-to-end- encryption. They use signatures known as digital certificates to authenticate and send encrypted emails that cannot be read by anybody except the recipient for which the mail is meant for. These signatures secure your emails while in transit against eavesdroppers. Although S/MIME encrypts your email contents, the metadata contained in your email header remains unencrypted. This still exposes important details about the sender or the receiver.
There are many other secure SMTP mail protocol alternatives that we will not discuss in this article. They all have their security flaws and to ensure that your email security is tight, it is important to use a Virtual Private Network that values your right to online privacy.