> On Mar 25, 2017, at 9:05 PM, Doug Barton <domain_name_t...@yahoo.com> wrote: > >> Postfix can be configured to try IPv6 before IPv4 (with >> smtp_address_preference) > > Regarding that option, I've never understood the warning in postconf(5). > Doesn't that feature provide precedence, not exclusivity? Or put a different > way, if the site cannot be reached by IPv6, won't postfix retry with IPv4?
See http://www.postfix.org/postconf.5.html#inet_protocols http://www.postfix.org/postconf.5.html#smtp_address_preference If IPv6 is explicitly preferred, rather than randomly selected as with: smtp_address_preference = any then an MX host with sufficiently many IPv6 addresses will never be tried over IPv4 because the number of connection attempts and SMTP sessions are limited: http://www.postfix.org/postconf.5.html#smtp_mx_address_limit http://www.postfix.org/postconf.5.html#smtp_mx_session_limit One or the other might be exceeded before any IPv4 addresses are tried. -- Viktor.
