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aioobe
Asked: 2018-06-22 10:39:47 +0800 CST

Sending mail with ssmtp over IPv4

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Apparently DigitalOcean blocks outbound SMTP on IPv6.

I can confirm this with telnet as follows:

⟫ telnet smtp.gmail.com 465
Trying 2607:f8b0:400d:c07::6c...
        > Waits for about 2 minutes before it continues <
Trying 173.194.204.108...
Connected to smtp.gmail.com.
Escape character is '^]'.

So it seems it falls back to IPv4 after a while.

Using telnet -4 works instantaneously:

⟫ telnet -4 smtp.gmail.com 465
Trying 173.194.204.108...
Connected to gmail-smtp-msa.l.google.com.
Escape character is '^]'.

I found that ssmtp has a -4 option too:

-4
      Forces ssmtp to use IPv4 addresses only.

For some reason however, I get the same result with and without -4. ssmt times out (or sometimes succeeds in sending the email after ~2 minutes).

My question: Why doesn't -4 work, even though telnet -4 can connect?

Alternatively: How do I best debug this? Is there some netcat command or something that I can use to confirm that ssmtp is really trying to connect over IPv4?

smtp

0 Answers