Hi..on one of our zimbra servers has lots of POP/SMTP clients using outlook. During the peak hours outlook is showing the following error

Task 'mail.domain.com' - sending' reported error (0x800CCC67):' The server responsed: 421 4.7.0 mail.domain.com Error: too many connections from xxx.xxx.xxx.xxx

i had setup IMAP and POP3 threads to 200 each and i have increased that to 400 each. can the above error be due to these limits. i have also increased the mysql connection and pool to 100

If the above settings are not for 421 error then where are the settings for these. Dose zimbra check number of connections per IP? As error message suggest.
* All the 400 users who access this zimbra server from their office are behind office firewall and Zimbra only see 1 IP from where all these connections are getting made