Describe the bug

A customer reported that they got the same email over and over again on the same day. The issue is that we simply retry all recipients when a single one fails, causing lots of emails to be sent.

E.g. imagine that a subscription has 3 recipients: a, b and c. If the recipient b fails, then we retry again a,b,c, causing a to receive the subscription twice

To Reproduce

This is a theoretical reproduction since I'm not able to repro unless I cause a failure to the email gateway:

1. set up metabase with an smtp server
2. make a dashboard and a subscription adding 3 recipients
3. when you send the test subscription, cut the connection to the email server

you should see the issue and A should have 2 emails, while b and c only one

Expected behavior

We should keep track of every recipient to send the emails only once

Logs

NA

Information about your Metabase installation

v50

Severity

P1

Additional context

Why did emails start failing on v50 more?