What does this mean? The system has timed out sending email (Case ID: -1) from SMTP Server

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The system has timed out sending email (Case ID: -1) from SMTP Server 

What does this mean?

This is not necessarily a critical error but it is included in the log anyway to advise you that Desk was not able to send an email to one of your customers. 

This can happen from time to time when there is no actual case number, but the SMTP server was not able to deliver an email (ie: an auto acknowledgement) for any reason, (sometimes the user's mailbox is full).

You can always double check your outbound mailbox also to verify if there are any emails queued up that are stuck. Go into the Admin, Channels, Email, Outbound Mailboxes to have a look.

I hope this helps. Have an awesome day.