Allow VRFY Command
The VRFY command allows external clients to verify that a user
exists in your messaging system. If enabled, VRFY can pose a
security risk because it allows external users to anonymously
request verification of user names. For example, if spammers
want to find out the usernames in your company, they could query
the system with a series of user names until the system verified
a valid username.
Allow EXPN Command
The EXPN command expands a group name upon request and lists all
the user names in that group. This command is also considered a
security risk because it allows spammers to anonymously request
group membership lists. For example, if a spammer requests to
expand a system-wide group such as Everyone, the SMTP Agent will
return the complete membership list which is, essentially, every
username in your organization.
Verify Addresses on Receipt
By default, the SMTP Agent accepts all incoming messages and
places them in a queue for address verification processing as
resources are available. This facilitates rapid message
processing. If you want the SMTP Agent to perform address
verification before accepting messages into your NIMS system,
select Verify Addresses on Receipt.
Send ETRN
The SEND
ETRN command requests a remote server to send any messages it
has queued for your messaging system. This option is primarily
for organizations with dial-up Internet connections.
Accept ETRN
The
ACCEPT ETRN command allows a remote server to request queued
messages. If enabled, the SMTP Agent responds to this request by
sending any messages it has queued for that system. Accept ETRN
is the only SMTP flag that is selected by default.