To be able to send emails for comments/replies that you add via NeetoDesk, you need to setup mailbox integration with Outlook as mentioned in https://help.neetodesk.com/articles/connect-a-mailbox-with-gmail-or-microsoft.
However, if you are a Microsoft 365/Office 365 user, you also need to ensure that Client SMTP auth is turned on in the Microsoft account connected to NeetoDesk. This allows NeetoDesk to update/retain the SMTP settings it requires to send an email on your account's behalf. If it is turned off, NeetoDesk will not be able to send emails for comments/replies made by you.
Here's how to enable it in your Microsoft account: