Bernd Wurst writes:
in this case, is blocked via greylisting by the recipient.
With greylisting, a connection to the server gets successfully established, but, from the sender's perspective, the delivery attempt fails.
The reasons for the delivery failures are irrelevant. To attempt to immediately try some other server to deliver the unwanted mail, is considered to be, at least rude, if not abusive. Rather, it is expected that the sender would wait, and try again later.
In your last reply, you stated that Courier does not make a preference on IPv4or IPv6. This may be right, but that means that when a IPv6 ressource recordis found first, courier only uses IPv6. What I miss is a fallback to the otherstack if the connections fails for any reason.
Any attempt to use another IP address, being IPv4 or IPv6, only occurs if there was a failure to establish a TCP connection. Once a TCP connection gets established succesfully, the connection to the domain is deemed made. If the delivery attempt fails, this is not considered to be completely separate from any connection-related issues.
This problem comes up when a connection via IPv6 should be possible but something is broken. This could happen, but that's what fallbacks and backup MXes are for. I just think Courier should try all possible addresses.
It does. If the TCP connection fails on the network level. When greylisting is in effect, the TCP connection does not fail.
With multiple IP addresses/MX records for a given domain, you are saying "here are other IP addresses to try if there's a networking issue that prevents the connection to the first IP address". With multiple MX records at the same priority you are saying "use these in random order to attempt to establish a conncetion".
Once a connection is established, the connection is deemed to be complete. What happens later no longer bears any relevance on the issue of establishing a connection to the recipient's mail server. That part is already deemed to be complete.
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------------------------------------------------------------------------------ What You Don't Know About Data Connectivity CAN Hurt You This paper provides an overview of data connectivity, details its effect on application quality, and explores various alternative solutions. http://p.sf.net/sfu/progress-d2d
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