Emails do not send on certain hosts as HTML

[fpcorso]

General description

Certain hosts/servers will not send HTML emails from the WordPress installation. Our plugin, currently, only sends HTML emails and not plain text. This causes these sites to not send emails.

Expected behavior

The plugin should be able to send both HTML and plain text emails.

Actual behavior

The plugin only sends HTML emails which causes it to not work on certain tech stacks.