To give some background, this error code is a catch-all error that gets thrown when we attempt to make an SMTP connection to your inbox using the credentials you provide, but get denied due to server connection issues. If you are positive you have SMTP Auth + IMAP access enabled under your inbox, this is most likely due to some type of DNS configuration issue while getting setup with your Workspace under your domain. Also this error can get thrown due to issues on Microsoft's end as there might be a temporary issue with the Exchange servers.
If you are receiving this error, confirm that you are able to send/receive mail correctly by attempting to send out an email, and email/reply back to that email you have sent yourself. If you are able to do this, than most likely your DNS configuration is fine.
If you are unable to send/receive mail correctly, most likely this error is getting thrown due to not fully configuring your workspace to your domain/subdomain, so you will need to identify what you still need to setup for this workspace in order to get it connected to the platform.
The quickest way to resolve this issue is to get your inbox setup with 2 factor authentication and create an App password for warmup inbox. The reason why we suggest this solution is because it allows us to make a much more stable connection to your inbox and will resolve this issue if there is not an issue with Microsoft's servers. We have created a guide on how to get setup with this, step by step found here.