587 is the standard port for IMAP (incoming) servers and 993 is the most commonly used port for the SMTP (outgoing) server. Unchecking the auto manage option in Mail will reveal the actual port in use for that account. However, not all email providers adhere to the standard incoming and outgoing port settings. A few providers, like AOL and Yahoo, have in the past been somewhat notorious for using non-standard settings (there are historic reasons for that relating to ill conceived legislative efforts) but I don't know if that is still true or not. The preferred incoming and outgoing port settings should be available on your provider's web. I would be surprised if NordVPN had problems with almost any email provider but if yours is too far off beat anything is possible.
