Spammers commonly abuse compromised or newly-created cloud servers to send bulk unsolicited email over Port 25, so many providers block it universally as a precaution, regardless of the individual customer's actual intent.
If you're running your own mail server (Postfix, Exim, Postal, MailWizz, PowerMTA and similar), you need Port 25 open for outbound delivery unless you route through a third-party SMTP relay instead.
Port 25 is the network port SMTP traditionally uses for server-to-server mail delivery; SMTP is the protocol itself.
To reduce spam originating from their network, since compromised or abused accounts have historically used it for bulk unsolicited email.
No, if relaying through a service like Amazon SES or SendGrid, that provider handles delivery, and outbound Port 25 from your own server isn't required.
Yes, Port 25 is open by default for legitimate opt-in email marketing and transactional sending.
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