S/MIME Signature corrupted by Lotus Notes IMAP

Using Mozilla to copy Email between an IMAP system and Lotus Notes, I got an issue with S/MIME signed messages. Actually, it look like Lotus Notes change the encoding of some part of the Email:

< Content-Transfer-Encoding: 8bit

to

Content-Transfer-Encoding: quoted-printable

Obviously, the digital signature will not be valid under this condition.

My Person document have the flag:

Format preference for incoming mail: Keep in senders’ format

Any others place to set the 8bits encoding on?