According to the internet specification,
when a mail server receives a "400-level" error, it must
queue the e-mail message and try later to deliver it. For
legitimate e-mail, this process is standard and mandatory.
Properly configured mail servers will redeliver their messages
appropriately and greylisting should not represent a delivery
challenge to them. Because SPAMmers send hundreds of thousands
of e-mails per day to addresses they do not know to be working,
they generate a large number of bounced messages. Acknowledging
server responses for these messages, storing the messages
on a server for some period of time, and redelivering
them again represents for SPAMmers a resource-intensive
process that might very well not return sales of their products
or services. As a result, they intentionally misconfigure
their mail servers. By requiring that every incoming e-mail
message to the University originate from a properly configured
mail server, most SPAM is filtered.
Note: Some SPAMmers have started to configure
their servers according to specifications, and therefore
some SPAM may continue to enter the University, but at a
dramatically reduced rate.
No. There are many reasons to assume that unencrypted
e-mail is not private, but greylisting is not one of them.
The SMTP servers only record information that is used to
deliver every e-mail message, and this information is captured
before the content of the e-mail is received. However,
e-mail is delivered across a vast network of servers on
the internet, any of which can potentially capture and copy
the data passing through them. You might never have an indication
that your e-mail is being read in cases like these.
There is no reason to assume that all - or even many -
e-mail messages are being read in this way by malicious
internet hosts. If privacy is a concern, there are several
simple encryption methods that you can use to protect your
e-mail messages within your email client software.
E-mail affected by greylisting will be delayed a minimum
of 10 seconds. This is the delay interval required by the
SMTP servers in order to prevent immediate redelivery by
already-connected SPAM servers. The message may be redelivered
without challenge by the servers for up to 12 hours. After
12 hours, the original record of the message is destroyed
and the challenge/redelivery process must begin again.
Internet specifications suggest that messages temporarily
refused be redelivered within 4 hours, and most servers
are configured to retry in far less time - often on the
order of 5 minutes. The specific delay will depend on the
configuration of the sender's e-mail servers
If the e-mail message is not received within 12 hours,
it is possible that the sender's e-mail server is not configured
according to internet standards. Contact
Technical Support for help determining if this is the
case. Copies of the headers from the bounced e-mail message
will be necessary for diagnosis of the problem. (Click
here to see MMI's instructions for viewing complete mail
headers in a variety of mail clients.)
There are several resources available to get more information
about greylisting. Here are some:
