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I added 653 users to the (previously empty) queue, which is set to 3 emails per 5 minutes. It sent the correct number for the first 30 minutes, then started sending 2 per 5 minutes, then down to 1 per 5 mins. After sending 162 of the 653 it stopped completely.
Are there any known clashes with other plugins that use the scheduler, the only one I have that looks likely is UpdraftPlus?