A 220 greeting and successful EHLO show that an SMTP service is listening and willing to describe its extensions. The capability list can include message-size limits, supported authentication mechanisms, enhanced status codes, and STARTTLS. This tool reports those lines but does not attempt any mechanism that needs a secret or changes server state.
On port 465 the connection is encrypted from the start. On 587 and 2525, the first EHLO occurs in clear text so the client can discover STARTTLS; after a successful upgrade, the client sends EHLO again because capabilities can change inside the encrypted session. Credentials should never be submitted when encryption was not negotiated.
Inbound and outbound servers answer different questions
MX records identify servers that receive mail for a domain. A submission hostname is the server a user or application connects to when sending, often after authentication. Test inbound routing with the MX lookup; inspect the host's A and AAAA records with the DNS lookup.
This diagnostic cannot validate a mailbox, password, relay permission, spam placement, or final delivery. Those steps require authorization and, in the final case, an actual message observed at the destination.