Attach whole certificate chain when signing e-mails using S/MIME

Hello,

I'd like to sign e-mails using S/MIME and the iOS mail app.

Therefore I installed the certificate containing my private key (issued by certum.eu) as a profile and activated S/MIME in the mail account settings (iOS settings). The certificate is selected there properly and the option to sign e-mails automatically has been enabled.

When I send e-mails now, they get signed - but not correctly: some e-mail clients say the signature was invalid.

When I send and sign e-mails with Outlook for Windows using the same certificate all the e-mail clients recognize the signature as valid.

The problem seems to be that the iOS mail app does not attach the whole certificate chain to the signature. Some e-mail client thus cannot validate it. (Windows for example is enrolled with certum's root CA by default but without the required intermediate certificates.)

The certificate I imported into iOS DOES contain all intermediate certificates (and they are listed in iOS when I retrieve the certificate's details there) and iOS says the certificate is 'trustworthy'. So all information needed should be available on the device.

How can I make the mail app behave like Outlook? Thank you very much in advance.

Best wishes