When I edit the contact form, in the bottom part of the page I can edit the error messages that may occur. I have modified the original text and translated them to spanish, so I don’t remember exactly the original text, but it’s the last one on the list, that goes something like: “Akismet has considered this mail as spam” and the actual error says something like “Sending mail failed. Please try again later or contact the administrator”
The site is on an intranet, so it’s not available outside our VPN, sorry.
