Digital signature - SMIME breaks using UM

[belchfireid]

The signed email arrives at the recipient as unsigned, with an attachment (smime.p7s). Turning off UM restores the signature function correctly.

Mac OS: High Sierra
Email client: Apple Mail

[Tekkle]

Same here with OSX 10.12.6, Mail 10.3 and UM 3b19. This ist really annoying, because I have two other machines (OSX 10.10.1, Mail 8.1, UM 2.1.5 and OSX 10.10.5, Mail 8.2, UM 2.1.6) and everything is working well on them. Would appreciate it very much, if there will be a solution for later versions in future. - BTW: Are there any alternatives to format the font-face in Mail to be "Outlook compatible"?