[exim] One user can't authenticate

I have one user who can't send email. She can receive it via dovecot,
she can log in, but exim refuses to accept her credentials.

From Mozilla Thuderbird:
2013-11-02 14:48:09 plain_server authenticator failed for
mail.seiner.com [192.168.128.2]: 535 Incorrect authentication data
(set_id=noriko)

From android phone:
2013-11-02 14:53:45 plain_server authenticator failed for
android-f16cd01f484477db.seiner.lan (infraware.co.kr) [192.168.128.131]:
535 Incorrect authentication data (set_id=noriko)
2013-11-02 14:53:58 TLS error on connection from
android-f16cd01f484477db.seiner.lan (infraware.co.kr) [192.168.128.131]
(send): The specified session has been invalidated for some reason.

I am stumped; all other users work fine, and I can't find anything that
is unusual about her account.

It's not an MUA issue as it happens from both an android phone and a
desktop.

Where can I look? AFAIK exim uses the standard linux authentication,
and this user can log in, retrieve emails from dovecot, but can't send
through exim. :headscratch: