Cisco Cisco Email Security Appliance C170 사용자 가이드

The SMTP server may return the following responses:

2xx: When an SMTP code starting with 2 is received from the call-ahead 
server, the recipient is accepted. For example, a response of 250 allows the 
mailing action to continue. 

4xx: An SMTP code starting with a 4 means that a temporary failure has 
occurred in processing the SMTP request. A retry may later be processed 
successfully. For example, a response of 451 means the requested action was 
aborted or there was a local error in processing. 

5xx: An SMTP code starting with 5 means a permanent failure in processing 
the SMTP request occurred. For example, a response of 550 means the 
requested action was not taken or the mailbox was unavailable.

Timeout. If no response is returned from the call-ahead server, you can 
configure how long to attempt to retry before a timeout occurs. 

Connection error. If a connection to the call-ahead server fails, you can 
configure whether to accept or reject a connection for the recipient address.

Maximum number of recipients to be validated in a single 
SMTP session. 

Specify between 1 - 25,000 sessions.

Maximum number of connections to a single call-ahead 
SMTP server. 

Specify between 1-100 connections. 

Size of the cache for SMTP responses. Specify between 
100-1,000,000 entries

Time-to-live value for entries in the cache. This field 
defaults to 900 seconds. Specify between 60 - 86400 
seconds.