If the visitors don't put in their email-address, it would be a one-way-communication:
The message entered in the contact form would go to the administrator(s) of the board (to the email-address entered in the acp) by email, but nobody would have a chance to answer that email, because nobody knows where to send it.
Sure, you can still use you preferred email program (like thunderbird, gmail, ...) to contact the board administrators: But the email program will give the senders address to the receiver for the same reasons - to answer the mail. If the sender doesn't want a answer he can use a dummy or invalid address (like
So, for clarification, you can extend the description in the language files, eg. "The email address you entered will be used to give the receiver a possibility to reply to your message".
And in your documents about privacy you can enter a sentence similar "email addresses entered in our web form (eg. contact form) will be used exclusive to answer the communication. It will not be used for other purposes and removed after the communication ended, the latest 6 month after the last contact via that email address. It can be stored longer in case of other legal reasons" (IANAL, just a example ...)