Schedule workflows on a list is unreliable if you have a lot of entries. Depending on how many users you have, I would recommend one of the following:
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Instead of scheduling your workflow on a list, use recursive workflows
[New Feature] Scheduling API workflows can now be done recursively
This is more reliable as it sends the mails one by one, but also takes more time if you have a lot of users. -
Alternatively, find an email api that allows you to send a batch of emails with a single api call. I think Postmark allows up to 500 recipients in a single call (which you could combine with the recursive workflow described in step 1 to send batches of 500 mails).