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I have my email provider that provides SMTP server. My Internet provider also provides SMTP server. Which SMTP server is better to use in this situation?

Is it true that internet provider might reroute SMTP sequests to it's SMTP server?

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I have my email provider that provides SMTP server. My Internet provider also provides SMTP server. Which SMTP server is better to use in this situation?

These days – always the email provider's.

For example, the domain might have SPF or DMARC policies set on it, requiring all mail from that domain to be sent through specific servers.

Your ISP's servers wouldn't be in the provider's SPF list, nor would they have keys for DKIM-signing the message (if required), so the messages would be marked as spoofed and get a much higher spam score.

Is it true that internet provider might reroute SMTP sequests to it's SMTP server?

Technically they could, but it would cause many problems for the customer.

Note that nowadays "client-to-server" SMTP (injection) and "server-to-server" SMTP (relaying) use different TCP ports and enforce different policies, e.g. the C2S ports 465/587 always require TLS & authentication while the S2S port 25 of course never does.

So if the ISP tried to block or redirect port 25, you shouldn't be affected, because your email app shouldn't be using it anyways.

But if the ISP tried to redirect ports 465/587, that would be a massive problem because it would look as if the ISP was trying to steal the users' email credentials (and of course it would be noticeable, because it would result in TLS certificate checks failing).

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