Cannot verify server identity and not trusted certificate on iPhone mail

I happen to have a problem with one of my mail addresses.

From that address I cannot send mail.

When I try to do that, it says: “Cannot verify server identity” and when I look into the details I see the message: “not trusted certificate”

I can imagine what the problem is, as the mail program sees this, it is dangerous to act. On my iPad I have the same problem, but… over there you go into these details and you can set the certificate to trusted by yourself. As I know the certificate is a good one, though apparently not recognised by means of the Apple implementation of the certification check.

While I have the opportunity on the iPad to control this myself (like on the mac), this appears not to be possible on the iPhone… which is very inconvenient. Apple has not implemented this possibility.

Does anyone know how to solve this problem on the iPhone?

Thanks for your comments in advance.

Jim