[Dovecot] Not an IMAP4 Server?

[pod]

>>>>> "JG" == John Gray <gray at agora-net.com> writes:

    JG> I've been testing alpha4, and every once in a while my client
    JG> (Thunderbird 1.5 RC1) tells me the dovecot isn't a valid IMAP4
    JG> server.  This is usually (maybe always) when I first bring up the
    JG> client up.  If I just ask the client to get the mail again, it
    JG> does so happily (without any errors).

Clutching at straws but do you have

login_greeting_capability = yes

in dovecot.conf?

I have encountered at least one client that would not talk to dovecot at
all without this option because the client wanted to see IMAP4rev1 in the
greeting.

Last time I looked I couldn't find any explict requirement for this in the
various RFCs though.