I’m having the same issue as OP.
For us, some of the contact forms are making it to the designated email, some are not. I have no idea what the root cause of this is.
Takayuki Miyoshi, what specific questions should I be asking our host for in the support inquiry? Also, when you say host I’m assuming you’re are referring to our website hosting provider?
Lastly, all of our form submissions are making it into the ContactFormDB plugin database, which is how we were able to identify this issue in the first place.
Thanks