Invalid Mailbox Syntax Breaks Form

I was using the following in my “To” field:

[Distributor1], [Distributor2]

to optionally email one of two email addresses, by setting one blank based on other form choices.

Now the help says:

“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.”

But having to do that breaks the previous functionality. How can I restore this to working condition?