As it says at the end of the first sentence: mailbox unavailable. I guess you entered the wrong mailbox data.
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While this might be a mailbox issue, an internal error mean that this error needs to handled differently.
I’m placing this into bugs.
Meanwhile @Lari, please speak with your email host in order for this to see whyis the mailbox unavailable.
Did you ever send anything in Manager with Mailkit? Please show your SMTP settings (screenshot).
Mailkit.dll is used by Manager.
Thank you, so @Lubos recent change of smtp service is now using NuGet Gallery | MailKit 4.2.0
I made it so the error message is less intimidating in the latest version but generally speaking, this is error response from SMTP server you are connecting to.
It’s not originating from Manager itself.