I'am using a valid s/mime certificate to sign my emails on my Mac Book Pro and my iPhone XS. When I sign the email from within the mail program on Mac Book Pro, the signature contains the certificate of the issuer (intermediate) of my certificate too. When I sign the email on the iPhone, the issuer of the certificate is not added to the signature. For that reason the email is displayed on the iPhone as not valid. The same problem in MS Outlook. The signed emails from Mac Book Pro are displayed correctly as valid. Why doesn't add the iPhone the issuer certificate to the signature?
Signed email on MBP (intermediate certificate is included):
Signed email on iPhone (intermediate certificate isn't included):