Our domain email address started receiving blank spam emails from our contact form (this is known from the header). They are from seemingly random email addresses and have no content. They are received roughly every 5 minutes with occasional 45 minute breaks. The form works correctly and I've tried both Askimet and Honeypot plugins (both together and separately) to no avail. The contact form (Contact-Form-7) has required fields. I believe Honeypot is failing due to the blank content and the hidden field not being populated. Here is an example of what the email contains:
From: WordPress [mailto:info@domain.com]
Sent: Thursday, April 13, 2017 3:47 PM
To: info@domain.com
Subject: contact form from domain.com
Name: 58f0002d1ad5f 58f0002d1ada9
Email: randomname@random.com
Subject:
Message:
The random name & email services are from various providers (billy@hotmail.com, kelly@yahoo.com, jim@gmail.com, etc). The Name field contains a seemingly random string (58efeaa9252e7 58efeaa92532d, 58efeaaf3099c 58efeaaf309da,58f0002d1ad5f 58f0002d1ada9, etc)
What is responsible for this particular type of spam?
What is the actual goal of blank spam email?
Is there another plugin or workaround that could filter the spam besides captcha?
Honeypot will not catch blank email spam as noted above.
Invisible reCaptcha will prevent this particular type of spam.
As far as possible motivation(s)/reason(s):
1.The spammers send the e-mail, along with a million others. Some bounce, some don't. They now know which e-mail addresses are still current.
2.Some people, amazingly, actually reply, even if it's just to say "Unsubscribe me!". Bingo, the e-mail address is current and there's a person behind it, and that person actually opens e-mails sent by the particular send address they've used.
3.Sometimes the content isn't as empty as it seems. For instance, many spam messages contain a one-pixel-by-one-pixel image with a trackback url; as soon as you view the message, they get pinged that this particular pixel was viewed - Bingo as above. That's the reason many modern mail clients warn you about images in messages and suggest you don't view the images unless you trust the source.