Invalid mailbox syntax is used.

This type of error is seen in the To or From field within the Mail tab. It may also be seen in the Additional Headers field if you specify mail header fields that expect a mailbox value (Cc, Bcc, or Reply-To).

A mailbox is typically an email address. The following is an example of a valid mailbox:

Another mailbox format consists of a name and email address surrounded by < and >, like this:

John Smith <>

This is an example of an invalid mailbox (because the email address isn’t surrounded by < and >):

John Smith

If you have multiple mailboxes, use a comma (“,“) as a separator:

John Smith <>, John Smith Jr. <>

You can use mail-tags in those fields, but be careful. When you use a mail-tag in an email address section of a mailbox, ensure that the mail-tag is always replaced by an email address.

Suppose you had the following content in the To field:


It’s all right if you have [email* your-email] form-tag in the form template.

[email* your-email]

Because the form-tag type email* indicates a required email input field, the corresponding mail-tag [your-email] will always have an email address value.

If it’s not email*, you’ll see an error because [your-email] can be empty or a non-email value. These are examples of bad form-tags:

[email your-email] // Email, but not required 
[text* your-email] // Required, but not Email 
[select your-email "" ""] // Not required

Just another contact form plugin for WordPress. Simple but flexible.