IOS mail SMIME signing

I try to use (GlobalSign PersonalSign 1) S/MIME certificate for email signing and encryption.

This works well on MacOSX Mail (10.13.1). But when I send an email from my iPad or iPhone (with the certificate, as I hope, properly installed), the S/MIME signature is marked faulty from IOS itself as well as MacOSX.

OX Mail tells me: "Unable to verify message signature", and "The digital signature for this message is incorrect. The message may have been tampered with or corrupted since being signed...".

It does not help to click the button "Messages from... are valid if signed...".

Decryption works well, even in IOS, and IOS thinks mails from OSX signed using the same certificate are indeed valid. Just the mails sent and signed on IOS are considered tampered with.

The IOS mail in ACSII looks like:

X-Mailer: Zimbra 8.7.11_GA_1854 (MobileSync - Apple-iPad7C3/1502.150)

--Apple-Mail-F380A000-6AD2-4676-8412-2F79FXXX39AD

Content-Type: multipart/alternative;

boundary=Apple-Mail-9B05CCA4-F5DB-4E48-9E0C-868A12358548

Content-Transfer-Encoding: 7bit

--Apple-Mail-9B05CCA4-F5DB-4E48-9E0C-868A12358548

Content-Type: text/plain;

charset=utf-8

Content-Transfer-Encoding: base64

DQprbnZrdWd1a2drdWdrdWdrdWdrd...

Whereas the OSX mail looks like:

Content-Type: application/pkcs7-mime;

name=smime.p7m;

smime-type=enveloped-data

Content-Transfer-Encoding: base64

Mime-Version: 1.0 (Mac OS X Mail 11.1 \(3445.4.7\))

Content-Disposition: attachment;

filename=smime.p7m

X-Mailer: Apple Mail (2.3445.4.7)

MIAGCSqGSIb3DQEHA6CXXXACAQAxgg...

Does this work for anyone? Sending Mail signed with S/MIME from IOS mail?