Stops sending after 55

Hello,

I have over 600 subscribers and the last 2 months only 55 emails are sending. I send manually and have used this plugin for several years. I’ve got the plugin set to send 500 at one time, but every send only 55 out of 600 send. I checked with the host and there are no blocks with them.

Can you point me in the right direction?
Thank you!

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