Add support and docs for SocketLabs email SMTP

backend/src/variables/index.ts

@@ -42,7 +42,11 @@ import {
   ACTION_DELETE_SECRETS,
   ACTION_READ_SECRETS
 } from './action';
-import { SMTP_HOST_SENDGRID, SMTP_HOST_MAILGUN } from './smtp';
+import { 
+  SMTP_HOST_SENDGRID, 
+  SMTP_HOST_MAILGUN,
+  SMTP_HOST_SOCKETLABS
+} from './smtp';
 import { PLAN_STARTER, PLAN_PRO } from './stripe';
 import {
   MFA_METHOD_EMAIL
@@ -101,6 +105,7 @@ export {
   INTEGRATION_OPTIONS,
   SMTP_HOST_SENDGRID,
   SMTP_HOST_MAILGUN,
+  SMTP_HOST_SOCKETLABS,
   PLAN_STARTER,
   PLAN_PRO,
   MFA_METHOD_EMAIL,