I sincerely appreciate your help, but I’m a bit confused.
You say: “you must make sure the "From" field uses an email address with a domain that's actually hosted by the server, otherwise it will most likely be flagged as a spoofing email.” However, if I do that, how will I be able to reply to emails sent by the users? They have to be able to enter their email address in the “From” field, so that I can reply to them, otherwise there’s no dialogue, just a user’s comment.
Also, I wouldn’t know how to add SPF records to my domains. Isn’t that something that ToCows (Site-5) is supposed to do? I have my domain names registered with them.
Thank you for your attention.