two things:
Please provide hardware information of the host Zammad runs on, slowness might be a hardware related issue when you have too many agents and not enough power (in hardware terms). Also it would be good to know how this slowness appears to you, to hunt down the issue for this better.
Please also run the following commands and give us the output:
zammad run rails r 'p Delayed::Job.count'
zammad run rails r 'p Delayed::Job.first'
zammad run rails r 'p Delayed::Job.last'
As @svnr-dvnkln already said, Authentication failed is a response from your E-Mail-Server.
You can check affected mailboxes either inside the mail channel (failed mailboxes will be shown with different “dot color”:
You can also check each mailbox and it’s error state via:
zammad run rails r 'p Channel.fetch'
Above will issue a fetch process and return issues and well… what Zammad did to improted stuff.
Last but not least regarding notifications delay:
As I stated in my first point regarding hardware, I guess your hardware or mail structure is having trouble. If your mail structure is totally fine, it will be Zammad related. Please note that outgoing mail and notifications are Background jobs and might be sent out later, if Zammad is missing the steam from CPU.
That’s really odd, maybe your mail server logs give a hint why the server responded that way during the time this appears in the logs? This shouldn’t happen too often. I am using over 20 mailboxes in my private setup and have never seen this issue.
Your load averade and CPU looks totally fine to me.
You can try to restart the Zammad-Service. If you want to make extra sure you’re not having a Zombie, just stop Zammad and search for scheduler and puma in your process list and kill those (if they’re still there after stopping the service).
Ohter than that, right now I’m not sure how to help you further.