Why won't emails send through a VPN, resulting in an SMTP error?

MAIL SERVICES BLOCK SENDING FROM SUSPICIOUS DATA CENTERS TO COMBAT SPAM.

Incoming mail works fine with encryption enabled, but outgoing messages are often blocked. Mail servers like Mail.ru, Yandex, and Gmail close mail ports for tunnel IP addresses to prevent spam bots from sending millions of malicious emails.

Protecting Mail Gateways from Mass Spam

The SMTP email sending protocol (ports 25, 465, or 587) is extremely sensitive to sender reputation. When thousands of users share a single exit node, mail providers add that IP range to spam blacklists: incoming mail continues to work, but sending outgoing messages from a local mail client is blocked completely.

How to Restore Email Sending

add your email client or webmail to the split tunneling exception list
switch to sending emails through the official web interface in your browser instead of a mail client
change the server to a less congested location with a clean IP address reputation
use port 587 with mandatory STARTTLS encryption instead of the standard port 25