Hi @lizard, there can be several reasons for this happening, I am sorry to hear about that.
Webflow sends out the notifications with no issues, so the place to check next is probably in the mail server policies together with the support group that helps you with mail issues.
My first suggestion would be to check if there is some kind of mail policy on the mail server, which does not allow emails to be sent from automated mailers.
Sometimes mail may be rejected by the mail server if the policy states that a certain number of emails with the same subject line, body text, sender etc are received, to reject that.
I would check the mail policies, and see if there is a whitelist that you can add the sender:
Another solution or workaround, is to create a dummy gmail account, add that to the list of emails to notify on form submission.
Next, setup a forwarding from that gmail account to the target email account. This way the email comes from a real user email, not an automated mailer.
I hope one of these solutions will help ! If not, you may just have to change the target email address to one that works. Usually a gmail address works.