It can be anything! An apex, subdomain, or even a URL.

Results forsmtp.mail.ziggo.com

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Location

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Name	TTL	Value
smtp.mail.ziggo.com	300	185.136.64.71 / Flowmailer infrastructure & services
smtp.mail.ziggo.com	300	185.136.64.72 / Flowmailer infrastructure & services
smtp.mail.ziggo.com	300	185.136.65.71 / Flowmailer infrastructure & services
smtp.mail.ziggo.com	300	185.136.65.72 / Flowmailer infrastructure & services