Send Grid failed to Send email

There was a problem while sending a test email. Related debugging output is shown below:

Versions:
WordPress: 4.9.6
WordPress MS: No
PHP: 7.1.18
WP Mail SMTP: 1.2.5

Params:
Mailer: sendgrid
Constants: No
Api Key: Yes

Debug:
Following RFC 1341, section 7.2, if either text/html or text/plain are to be sent in your email: text/plain needs to be first, followed by text/html, followed by any other content. – content;