SMTP username field does not stay blank

My work SMTP server does not require (and does not want) a username. When I open the IMAP Account Settings dialog for my work account, the SMTP Server Username field is filled with a weird UTF-8 looking character (see attached screenshot).

If I remove the character, sending emails through the SMTP server works, but the character reappears the next time I open the dialog.

If I leave the character in place and accept the changes, next time I try to send an email through this SMTP server I get the following error message:

Enter password for “Ôøø@admmail.lmd.polytechnique.fr”