Last night I started sending an e-mail about the address change to users who haven't logged in since it happened, but I made a mistake. I had to close the process and then I found that e-mails were being sent in a random order so I had no way to "resume" from the right position without sending people a duplicate.
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Great news! I found a way to extract the e-mails that already received the message from the e-mail service provider, so now I'm resuming the sending without worrying about bothering people with a duplicate.
