Question
We frequently find that spammers are reverting to basic techniques in the hope of fooling Anti Spam appliances. The idea is to send a message to a local mailbox claiming to be from a local user, often claiming to be sent from your own e-mail address. As the message looks to come from a local sender, there is the chance that an Anti Spam appliance will see this as a trusted sender and therefore allow the message through as usual.
Answer
GMS Anti-Spam has a mechanism known as "Local Clients". This setting aims to prevent spammers connecting to your server and ultimately sending a message to a local user claiming to come FROM either your own e-mail address or that of another local user. This setting can be enabled on a domain level basis – GMS Anti Spam > Connection > Local Clients.
Entering an IP address or range of IP addresses into the listbox will dictate which IP addresses can connect to your server and be able to send mail claiming to come FROM a user on your domain.
Please Note: Enabling this could have an adverse effect on roaming users that work externally to the server and need to first authenticate to GMS.
To prevent your server from rejecting legitimate senders, you can choose to enable "Check Local IP first" which will require GMS to look-up your "Local IP" range to see if the connecting IP is trusted. This setting will also trust authenticated users who have used either POP/IMAP before SMTP or SMTP Auth to authenticate to the server. Leave "Check on connection" disabled.
Your Local IP range can be found under GMS Mail > Security > Local IP.
If one or more of your users have a Blackberry setup in a BIS environment to work with GMS, this may prove more challenging as this feature may require regular administration, particularly if Blackberry frequently change the dynamic IP address range of your users’ device. It may be worth contacting Blackberry and asking whether you can obtain a range of IP addresses that are likely to be assigned to the Blackberry device. If you are successful, enter these IP’s into "Local Clients" not "Local IP" as this would make you an open relay.
See Also:
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