WP8/Windows 10 mail app can't login, tries double auth

[Nagy, Attila]

Hi,

Are there anybody who could make WP8 (and W10 desktop clients it seems) 
devices working with Dovecot's POP3 server?

The conversation looks like this:
S +OK ready
C CAPA
S +OK
S CAPA
S TOP
S UIDL
S RESP-CODES
S PIPELINING
S AUTH-RESP-CODE
S USER
S SASL PLAIN
S .
C AUTH
S +OK
S PLAIN
S .
C AUTH PLAIN
S +
C <base64 auth string)
S +OK[space]
C USER username
S -ERR unimplemented

First I thought the extra space after that OK causes this, but removing 
it changed nothing.

So these devices try to log in with AUTH PLAIN, then even after a 
successful login they want to log in again with USER, which gets rejected.

The only solution here seems to be giving back -ERR to the client's AUTH 
command, which makes it try USER instead.
I'm not aware of any configuration options which makes this possible.

Any other ideas?

Thanks,