Smtp not sending messages

Replies: 1

It was working for sometime and suddenly, mails are not being sent. This is the error message I get.
What do I do?

Versions:
WordPress: 4.9.6
WordPress MS: Yes
PHP: 7.1.18
WP Mail SMTP: 1.2.5

Params:
Mailer: mail
Constants: Yes

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