pam+cyrus failed to authenticate

[sam wun]

Hi,

I got cyrus-imap2.2 and cyrus-sasl2.1.20 with saslauthd2 compiled in 
FreeBSD 5.3.
I can successfully login with the following cyradm command:
# cyradm -u cyrus --server gateway.mydom.com --auth plain
Password:
IMAP Password:
              gateway.mydom.com>
The log corresponding to the above cyradm command is:

Nov 16 06:06:43 gateway imap[73636]: badlogin: gateway.mydom.com [192.168.4.88] PLAIN [SASL(-16): encryption needed to use mechanism: security flags do not match required]
Nov 16 06:06:46 gateway perl: No worthy mechs found
Nov 16 06:06:50 gateway imap[73636]: login: gateway.mydom.com [192.168.4.88] cyrus plaintext User logged in

I can see there is some problem here eventhought cyradm login 
successfully, but the second log message indicated that cyrus is logged in.
Then, I also added bob at mydom.com user account using the cyradm admin shell.

I further test the cyrus server by adding user at domain.com to the 
imap.password file:
pwadd -a bob at mydom.com
# cat imap.passwd
abc.xyz at mydom.com:$1$OxTrXXu7$SPv0UCpp4BuyFGy6uQkBn1
cyrus:$1$EUHsnXCc$qpuk26X8VPQnIifMbnap6.
bob at mydom.com:$1$3gb6Wviv$0zrfF91CdEd3IlI7c62QQ1

But imtest failed with the following message:

Nov 16 06:05:16 gateway saslauthd[73020]: user not found in password database
Nov 16 06:05:16 gateway imap[73621]: badlogin: gateway.mydom.com [192.168.4.88] plaintext bob at mydom.com SASL(-13): authentication failure: checkpass failed

I searched google, but found not much useful information.
Can anyone tell me how to fix this problem?

Thanks
Sam