WD21 - email error : '550 Subject contains invalid characters.''

I'm using WD21 to send email to customers. Some of these email are rejected by the RECEIVERS provider with the following message

Remote Server returned '550 Subject contains invalid characters.'

The subject of the email is this : I can only think "ä" being the problem. Can anyone confirm that?
Do I need some kind of UTF8 conversion before sending?

I'm using this simplified code

MyEmail is Email
MySession is EmailSMTPSession
MySession..ServerAddress = etc (using company Exchange server)

Email.Subject = 'Bestätigung Termin Hausbegehung Deutsche Glasfaser'
Email.HTML = etc

EmailStartSession(MySession)
EmailSendMessage(MySession,MyEmail)