Chris, somewhere in your 1and1 control panel, presuming they hold the DNS records for your domain name, you will get access to the aname and mx records etc for your address.
In layman terms (and Im sorry if this is insultingly basic) its usually where you can configure subdomains - like developer.authorize.net as opposed to just authorize.net.
Within that area there should be a TTL specified against your records - that is a length of time in seconds that afterwhich the record is to be considered out of date, and therefore needing refreshed. A low TTL means that other DNS servers will constantly be contacting your hosting DNS server, needing refeshed. A high one means a low level of related traffic.
It also means though, that the external DNS servers refering to your domain (such as Authorize.net's) will be forced more regulary to update the records to your site - it is this effect that appears to be helping resolve the problem.
Here is a question for all you Jedi-server-admins out there - if we were all to set our TTLs to 1 sec (or at least a significant number of users) wouldn't that eventually cause more problems? If the DNS server being used to lookup the relay response domain started to grumble under heavy updating traffic - wont it impact just the same? (presuming this is the actual issue)
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