When configuring an SMTPAppender, an empty username value is treated as valid value.

Specifically:

<appender name="EMAIL" class="ch.qos.logback.classic.net.SMTPAppender">
  <username></username> 
</appender>

We use variable substitution, so it would actually be 

<appender name="EMAIL" class="ch.qos.logback.classic.net.SMTPAppender">
  <username>${SMTP_USERNAME}</username>
</appender>

 And sometimes we don't actually want to authenticate.

It would be nice if empty usernames were handled the same as if it were never specified.