Re: Really not prefer IPv6 MX

Sam Varshavchik <[email protected]> Tue, 03 Dec 2024 08:30:26 -0500

Newsgroups	gmane.mail.imap.courier.general
Message-ID	<[email protected]>

Alessandro Vesely writes:

> Hi Sam,
>
> as I have IPv6 addresses, I'd like to be able to (try to) send messages to  
> domains that have no IPv4 addresses.  However, I have no chance to set up  
> reverse PTR, as my provider itself didn't claim its reverse zone.  Hence, if  
> it's possible to use IPv4, I'd rather re-queue than use IPv6.
>
> I have ESMTP_PREFER_IPV6_MX=0 in courierd.  It doesn't seem to have any  
> effect, as I introduced it after having fared without it for a while and  
> didn't see much difference (similar preference is probably driven by  
> gai.conf).  I get sporadic rejects like so:

How recent is the code you're running? There were some issues related to  
that which were fixed in 1.3.7, but having said that:

ESMTP_PREFER_IPV6_MX=0 does not mean no IPv6 ever, it means that IPv4 is  
tried first before trying IPv6. So if there was a connection failure to the  
IPv4 address then the IPv6 address gets attempted, so in the outlined  
scenario you could still get an occasional failure, because of that.

If you have a limited number of problematic domains you can try to put them  
into esmtproutes, as a bandaid. The best, long term solution would be to set  
aside an IPv4-only smarthost, and use esmtproutes to direct the problem  
domain to the smarthost.

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