Unable to send test email

Hi,
I am not able to send test email through WP Mail SMTP and when I click to view the full error Log for debugging, this is what i get

Versions:
WordPress: 4.9.9
WordPress MS: No
PHP: 5.5.38
WP Mail SMTP: 1.4.1

Params:
Mailer: mail
Constants: No

Server:
PHP.mail(): Yes
Apache.mod_security: No

Debug:
Mailer: Default (none)
PHPMailer was able to connect to SMTP server but failed while trying to send an email.

I am newbie at SMTP server setup. Can you help?