Sender Address Blocked

Sender Address Blocked

Hello, 

I noticed that i cannot send any mail anymore from wordpress (smtp).

SMTP ERROR: Password command failed: 554 5.1.8 Sender Address Blocked
SMTP Error: Could not authenticate.CLIENT -> SERVER: QUIT

Is this account blocked? Can this be fixed and what causes this?
It is only used to send registration mails from a wordpress site with +- 20 mails/month

mail: info@berlaarsedansstudio.be

Thanks!