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

Results formail.nexion.dev

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2023-05-11T02:30:38.132	2023-05-11T01:30:37	2023-08-09T01:30:36	www.psu.pbv.mybluehost.me	mail.nexion.dev	C=US, O=Let's Encrypt, CN=R3
2023-05-11T02:30:37.854	2023-05-11T01:30:37	2023-08-09T01:30:36	www.psu.pbv.mybluehost.me	mail.nexion.dev	C=US, O=Let's Encrypt, CN=R3
2023-05-10T07:22:23.991	2023-05-10T06:22:23	2023-08-08T06:22:22	psu.pbv.mybluehost.me	mail.nexion.dev	C=US, O=Let's Encrypt, CN=R3
2023-05-10T07:22:23.891	2023-05-10T06:22:23	2023-08-08T06:22:22	psu.pbv.mybluehost.me	mail.nexion.dev	C=US, O=Let's Encrypt, CN=R3
2021-12-02T03:47:29.043	2021-12-02T02:47:28	2022-03-02T02:47:27	mail.nexion.dev	mail.nexion.dev	C=US, O=Let's Encrypt, CN=R3
2021-12-02T03:47:28.908	2021-12-02T02:47:28	2022-03-02T02:47:27	mail.nexion.dev	mail.nexion.dev	C=US, O=Let's Encrypt, CN=R3