This type of error is typically seen in the Subject or Message Body field within the Mail tab. It indicates that the field content is empty or only contains mail-tags which could be replaced with an empty value.

Be careful not to make those fields empty. It not only makes the mail look like spam, but it can also cause an error in the mail-sending function on some hosts.

If you use mail-tags in the fields, ensure that it’s not blank even after the mail-tags are replaced with user inputs.

Suppose you had a mail Subject field containing only a mail-tag, such as [your-subject]:

Subject: [your-subject]

As long as the submitter inputs something into the corresponding “your-subject” form field, the input replaces the [your-subject] mail-tag; this guarantees that the mail subject is not empty.

However, if the submitter leaves the field blank, the mail Subject would become empty too. You can ensure that the mail Subject field always has a value only when the “your-subject” is a required input field.

To avoid empty mail fields, add some text in addition to the mail-tags, like this:

Subject: You got a message "[your-subject]"

Or, make the corresponding form-tag required:

[text your-subject] // Optional text field => Not good
[text* your-subject] // Required text field => Good