Smtp Authentication Failed - error 535

My contact form doesn't work, so my host administrator suggested i should use the Wordpress plugin Postman to authenticate to your SMTP server.

I have installed it and configured it using the settings on your site.
Type : SMTP
Mail server: smtp.zoho.eu
port: 465
security: smtps
authentication: plain
username: <myemail>
password: <mycorrectpassword,Iswear>

Still, I am not able to send a test email to an address of mine. The error message is "Authentication Failed" and the session transcript with the server is the following:

220 mx.zoho.eu SMTP Server ready May 5, 2017 5:30:56 PM CEST
EHLO localhost
250-mx.zoho.eu Hello localhost (server.win58.papaki.gr (138.201.31.82))
250-AUTH LOGIN PLAIN
250 SIZE 53477376
AUTH PLAIN
334

535 Authentication Failed

My credentials are correct since I just typed both of them to login to my webmail.

Could you please help me with SMTP authentication to your server? Are there any extra settings I need to configure such as type of Security (SMTPS or STARTTLS) or Authentication (PLAIN or LOGIN)?
It's kind of urgent, since I need my contact form up and working.

Thank you in advance

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