why so many pop3 and imap ports ?

[JOYDEEP]

Dear list,

Kolab-bootstrap reports

KOLAB BOOTSTRAP

Check for running webserver on port 82
Check for running webserver on port 447
Check for running imap server on port 143
Check for running imap server on port 220
Check for running imap server on port 585
Check for running imap server on port 993
Check for running pop3 server on port 109
Check for running pop3 server on port 110
Check for running pop3 server on port 473
Check for running pop3 server on port 995

I wounder why so many imap and pop3 ports ? 4 imap and 4 pop3, any idea ?
thanks