Problem: can't send email through Wordpress contact form

Problem: can't send email through Wordpress contact form

I'm using a virtual server with Ubuntu 16.04LTs and a website with NGINX/LEMP combination.

The idea was that the site uses SMTP plugin to send user filled forms through ZoHo servers.
The plugin works perfectly with Gmail and several other e-mail systems, such as Exim.

I have successfully set up domain on ZoHo, verified it, SPF and DKIM. Those credentials and settings work perfectly OK from Outlook.

When trying to send e-mail through ZoHo mail, it cannot authenticate user in order to send mail.
I tried every combination of encryption/encapsulation and port.

The credentials I'm using are correct one (checked many times even reading and comparing backwards, just to be sure).

It seems like Wordpress connects successfully to ZoHo server but cannot authenticate the user.

Here is the mail log - parts of IP address are [HIDDEN]:
 
  1. 220 mx.zohomail.com SMTP Server ready July 13, 2017 4:36:49 AM PDT
  2. EHLO localhost
  3. 250-mx.zohomail.com Hello localhost ([HIDDEN].[HIDDEN].177.75 ([HIDDEN].[HIDDEN].177.75))
  4. 250-STARTTLS
  5. 250 SIZE 53477376
  6. STARTTLS
  7. 220 Ready to start TLS.
  8. EHLO localhost
  9. 250-mx.zohomail.com Hello localhost ([HIDDEN].[HIDDEN].177.75 ([HIDDEN].[HIDDEN].177.75))
  10. 250-AUTH LOGIN PLAIN
  11. 250 SIZE 53477376
  12. AUTH LOGIN
  13. 334 VXNlcm5hbWU6
  14. aW5mb0BhdXRvbWVoYW5pY2FyLmhy
  15. 334 UGFzc3dvcmQ6
  16. MUdBYDg5OHAkQXYsfFhs
  17. 535 Authentication Failed


Can you help me with that authentication and mail sending?

V.