Clarify MAIL_URL.

Author: abernix

source/environment-variables.md

@@ -27,7 +27,9 @@ Set this to however many number of proxies you have running before your Meteor a
 ## MAIL_URL
 (_development, production_)
 
-If you're using an external mail service like [Postmark](https://www.postmarkapp.com), [Mandrill](https://www.mandrillapp.com), [MailGun](https://www.mailgun.com) or [SendGrid](https://www.sendgrid.net), you can provide a SMTP URL for your Meteor app to use to send e-mail. For example: `MAIL_URL="smtps://user:pass@yourservice.com:465"`.  If the e-mail server is **not** using TLS/SSL or uses connection upgrade (e.g. on port `587`) then `smtp://` can be used in place of `smtps://` (the `s` is for "secure"). For more information see the [Nodemailer docs](https://nodemailer.com/smtp/)
+Use this variable to set the SMTP server for sending e-mails.  [Postmark](https://www.postmarkapp.com), [Mandrill](https://www.mandrillapp.com), [MailGun](https://www.mailgun.com) and [SendGrid](https://www.sendgrid.net) (among others) are companies who can provide this service.  The `MAIL_URL` contains all of the information for connecting to the SMTP server and, like a URL, should look like `smtp://user:pass@yourservice.com:587` or `smtps://user:pass@yourservice.com:465`.
+
+The `smtp://` form is for mail servers which support encryption via `STARTTLS` or those that do not use encryption at all and is most common for servers on port 587 and _sometimes_ port 25.  On the other hand, the `smtps://` form (the `s` stands for "secure") should be used if the server only supports TLS/SSL (and does not support connection upgrade with `STARTTLS`) and is most common for servers on port 465.
 
 ## METEOR_SETTINGS
 (_production_)