On 05/03/2017 14:15, Matus UHLAR - fantomas wrote:
does the mx0 has highest preference (lowest priority)?
If not, there's little point in using it - nolisting is supposed to catch
spambots trying to connect to your backup MXes, not to primaries.
No its not. Nolisting is to catch spambots that are firing off and
cannot wait or handle the idea of MX sequences properly due to their
'fire-and-forget' attitude. Most genuine mail servers would try the
highest preference (lowest priority) first and if not available/timeout,
drop to the next highest (a backup MX) and so on. Spambots dont want to
wait for the timeout of the first attempt to then lookup and try the
next on the list and intead just just bail out (time isnt on their
side). Occasionally there might be one that simply tries the last on
the list (the idea that it is a backup MX and often with less
protection) - and thats why its a good idea to put a dummy MX also in
this position (just like the first one).
I suspect the OP understands this and this is why he has it set as
such. The problem (if it exists) that Yahoo is not following protocol
to retry the next MX on the list is geniune and is one of the reasons
why some would say implementing Nolisting is dangerous (as in the risk
of genuine mail servers not configured and performing correctly and
simply returning mail back to sender). I must say I am VERY surprised
to find it is Yahoo though - and especially that it seems to be only
some of their servers. I doubt they know they have the problem and
perhaps should be reported to them.
