Mails to ressource mailboxes / SPF

(Kind of) continuing the discussion from Webforms.io Mail Server Information:

I keep having the problem, that form submissions to info@… mailboxes don’t arrive. Actually, I have the feeling the problem is becoming bigger. I run quite a few customer websites which loose business leads. So far nobody has filed for damages, but it’s a serious problem.

On Dec 3rd Dave from Webflow wrote to me:

(…)
In general mails that have resource mailbox names will have a higher likelyhood of getting bounced by our bulk mail provider. We cannot however guarantee that a mail will reach it’s destination, (…)
Whitelist the email address: no-reply@webforms.io on your email account and perform another test. (…)

This was helpful in some way, but that’s not really a solution. Especially because with customers I usually only get access to edit DNS info, no email administration.

Now a friend told me that somehow (yet have to read furthermore about it) I might be able to legitimate the webforms.io service by creating DNS entries, something with SPF. My understanding right now is, that the domain registrar somehow publicly announces that mails addressed its domain can come through webforms.io.

Does anybody have experience with this? Can anybody provide the info about the webforms.io servers, that I need to enter?