Grey Listing is a method of defending against spam messages. When grey listing is enabled,
Xeams will reject every incoming message the first time an email is encountered from a particular IP
address. Most legitimate email servers will retry sending an undeliverable message
where as spammers who want to send millions of messages do not retry.
Although using grey listing will eliminate a significant number of junk messages, there are some side effects that
you should be aware of before enabling this feature.
- Message delay - You will lose the instantaneous nature of email delivery. Since Xeams will temporarily reject incoming
message from a new IP address, most emails from new senders will be delayed. The actual time depends upon the retry interval of the
sender's server and could range from five minutes to hours.
- No delivery - In rare instances you may not get the message at all. This happens if the sender's SMTP server is old and
treats a temporary failure as permanent. In such cases, message will never be received. This could also happen if the retry attempts are
made after thresholds set in Xeams. See Greylisting Threshold below.
- Delivery from multiple IP - Some large companies use multiple email servers to handle out-bound emails. In such cases,
the retry attempt can be made from a different IP address. Since Xeams will consider the second attempt from a different IP, it
will put even the second attempt on hold increasing the delay even further. Once such company is GMail or GoogleMail.
Grey Listing Exception
Xeams provides a way to add certain domains that are exempted from grey listing. Consider adding well known domains
to this exception list. For example, gmail.com, hotmail.com, and yahoo.com. This way
messages from such domains will never be held in the grey list allowing messages from these domains to be received right away.
Note that adding domain names in this list does not mean the message won't go through any other filter. You can still get a spam
message where the sender's domain is gmail.com and Xeams will block such messages if the score is high enough.
To add domains in this exception list, modify
add the desired domains one per line.
Adding IP address to exception VS domain name
Many supporters of grey listing suggest white listing a set of IP address. Xeams on the other hand white lists domains rather
than IP. This is done for the following reasons:
- It is a lot easier to maintain domains instead of IP address. It is very common for IP addresses of the sender to change.
Additionally, there can be multiple IP address for a domain to white list. This increases administrative work since you will
have to frequently update the list of IP addresses.
- Even if someone forges the domain name of the sender, other rules such as SPF in Xeams will block that message.
Enabling grey listing
Follow the steps below to enable grey listing.
- Ensure you are using build 5745 or higher. Grey listing does not exist in older builds.
- Click SMTP Configuration under Server Configuration
- Select the tab for Advanced Configuration
- Check Grey Listing
Xeams maintains a special log file when grey listing is enabled. The name of this file is called GreyListing.log and is accessable
from the View Logs
sections under Tools
menu. This log contains following information:
- Date/time of the event
- State - this can be one of the following values:
- GREYED - Occurs when an IP address is first encountered.
- WAITING - Indicates sender's SMTP server retry attempt before Minimum retry interval has elapsed. See Greylisting threshold below.
- PASSED - When a second attempt from the sender's SMTP is received and is after the Minimum retry interval. Once you see this in the log, this IP address is automatically white listed and won't be grey listed any more.
- REMOVED - You will see this when an IP address never retried another attempt.
- Sender's IP address
- Sender's email address
- Recipient's email address
The following two parameters affect greylisting.
- Minimum retry interval - Number of minutes before a grey listed IP address is white listed by the system. This is set to 5 minutes by default. This
means the sender's SMTP server must wait at least 5 minutes before re-sending the email.
- Expiration interval - Number of minutes after which an IP is no longer kept in memory - meaning if a retry attempt is made after
this many minutes, it will not be white listed. Instead, it will be treated as a new attempt and will be grey listed again. This value is set to
600 minutes or 10 hours by default. Therefore, if an incoming email is retried after 5 hours, you will never get that message in Xeams.
Follow the directions below to modified these default settings.
- Locate server.properties file in $INSTALL_DIR\config folder. If this file does not exist, create a new one.
- Add the following lines:
- #Modify the Minimum retry interval to 2 minutes
- #Modify expiration interval to 5 hours
The grey listing feature affects every profile in Xeams.