Email Notifications don't support multiple to addresses

[gokulav137]

Currently each recipients email address would be send a separate email address.

So an email will be sent out for each recipient [email protected] and [email protected] for the following:

apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: another
  annotations:
    notifications.argoproj.io/subscriptions: |
      - trigger: [on-cert-ready]
        destinations:
          - service: email
            recipients: [[email protected], [email protected]]
spec:
  secretName: something
  issuerRef:
    name: letsencrypt-staging
  emailAddresses:
    - [email protected]

Proposition

I would like to propose adding a feature to support comma separated email addresses so that we can we can do the below:

apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: another
  annotations:
    notifications.argoproj.io/subscriptions: |
      - trigger: [on-cert-ready]
        destinations:
          - service: email
            recipients: ["[email protected],[email protected]", [email protected]]
spec:
  secretName: something
  issuerRef:
    name: letsencrypt-staging
  emailAddresses:
    - [email protected]

Here one email would be sent with only [email protected] as recipient and another with both [email protected] and [email protected].

This can be done by introducing the following change in https://github.com/argoproj/notifications-engine/blob/master/pkg/services/email.go#L77-L84:

to := String.Split(dest.Recipient, ",")
email := smtp.New(smtp.Options{
........
........
}).WithSubject(subject).WithBody(body).To(to[0], to[1:]...)

This shouldn't break any existing functionality. I have tested this out locally.

I can raise a PR if this seems useful.

Originally posted by @gokulav137 in argoproj/argo-events#2793 (comment)