O.K., you found it. Here it comes :)
telnet localhost 2000
Trying 127.0.0.1…
Connected to localhost.localdomain.
Escape character is '^]'.
"IMPLEMENTATION" "Cyrus timsieved v2.2.12-Gentoo"
"SASL" "NTLM LOGIN PLAIN DIGEST-MD5 CRAM-MD5"
"SIEVE" "fileinto reject envelope vacation imapflags notify subaddress relational comparator-i;ascii-numeric regex"
"STARTTLS"
OK
Seems like Gentoo's cyrus timesieved is listening there :) Thanks. How do I reconfigure JOnAS to another port?