It seems I have been a victim of e-mail spoofing as I received a lot of  undeliverable alerts on e-mails I never sent. Due to this I now get the following warning when trying to send my own mails:

Unable to connect SMTP server:smtpout.mail.zoho.com Reason:Authentication failed:554 5.1.8 Sender Address Blocked

I'm not sure how I can prevent others from using my address for their nefarious activities but would like to get the ability to send my own mails back. Please advise how to unblock mt account and how best to handle this in order to prevent the 'bad guys' to use my address to send out spam messages. 

Thanks in advance for your help.

Kind regard
Rodger