Email sent to customer from Drupal has RED LOCK icon in the email
I've read both your questions regarding this issue posted here. IMHO the red lock is not a thing you can overcome by encrypting the contents of your mail before sending it via an unencrypted channel.

This issue is not directly connected to Drupal, but to your Drupals mail gateway. Obviously your mail relay is not supporting TLS mail forwarding.

I do suppose you are sending the mails using Drupals built-in mail functions. These functions do use the PHP mailing functions, which by default are set-up to use your hosts mail services.

If you are working with a root server and installed your mail server yourself (e.g. postfix on Unix), you have to digg into its configuration and make sure it is set-up to use TLS when connecting to other mail servers.

When you don't have direct access to these settings (e.g. on a shared hosting package), you can overcome this issue by creating a mail account with a third-party email provider that is using TLS (e.g. gmail itself). With this mail account you will also get SMTP credentials to use for sending mails from your desktops mail client.

In your Drupal, you can install and configure the SMTP module to use these credentials for letting Drupal send mails via this external mail provider. Please note that some mail providers have flood mailing prevention in place that do allow a certain amount of mails each time/per minute only. So you might additionally consider using Queue Mail and a Cron job to send your mails in batches.

Edit:
Drawback of the second solution is, that the mails received by your sites visitors may have been flagged as "x-forwarded" or have the third-parties mail provider email address as sender. Some mail clients therefore may mark the mails as spam.

So another solution for shared hosting: Ask your hosting provider to change the default mail server settings.