[Dovecot] [Q] auth(default): plain(?, 11.111.206.99): Username contains disallowed character: 0x20

[JKL]

Dear all,
   
    A new user with Microsoft Express is trying to use IMAP SSL port
993. However they cannot log in.  Its only with them.
The logs details the following:

Feb 11 18:56:03 logout dovecot: auth(default): plain(?,11.111.206.99):
Username contains disallowed character: 0x20
Feb 11 18:58:07 logout dovecot: imap-login: Disconnected (auth failed, 1
attempts): method=PLAIN, rip=11.111.206.99, lip=62.58.61.184, TLS:
Disconnected

I think that the 0x20 is a space. However the user states that they are
using this format and that there is no space:
user.name at test.com

Does anyone know where I could start to look for this? 

Regards,S