Most residential and cellular ISPs (Comcast, AT&T, Verizon, T-Mobile) plus cloud providers like AWS EC2 block outbound port 25 by default. Reason: any compromised home PC or VPS could otherwise spew spam directly to remote MX servers, since port 25 is the SMTP server-to-server relay port with no built-in authentication. Blocking outbound port 25 forces mail through an authenticated ISP relay or provider SMTP (which can throttle abusers). Use ports 587 (Submission with STARTTLS) or 465 (SMTPS with implicit TLS) for mail submission instead — those are for authenticated client-to-server mail. Check port state with the port scanner.
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