Thanks for the link, was an interesting historical link. I've already sent my comprehensive e-mail to you (maybe you should check your spam folder).
I received your email, now!
It seems that the issue mainly comes from attached files. There is no option for this, unfortunately. On the other hand, it's an old email address I mainly use for my website, so it is quite exposed to spam nonetheless.
Thank you again for your review, I will put you on the hall of fame today.