Email signed with S/MIME not parsed properly on eM Client

Hello dear eM Community,

I’m opening this thread because I’ve been having a bug for a few weeks now and I can’t figure out if the problem is with my email client or with the Mailcow instance.

I currently use MailCow with the eM Client for Windows. So far no problems, however I noticed that I sometimes lost some emails (invisible in the list and when searching)

After extensive testing and discussion with the eM Client support (I am a Pro customer) forced to admit that they are also lost but here are my conclusions:

1. First of all I use S/MIME certificates to sign all my outgoing emails (this is an important point)
2. I use the thread view (or conversation) feature in eM Client to group my emails together.

I have noticed in these conditions that when I start a thread (with one or more people) and reply in that thread, from the 2nd email sent from my Mailcow box and signed with S/MIME, the emails start disappearing randomly … (at each new message in the thread the email disappearing can change…)

But the emails are actually present on the server side (and on the client), because when I disable the Thread View function the emails reappear correctly.

My first reflex was to contact the eM Client support to inform them but during the tests with them I realized that this problem only occurred with MailCow. No problem with a Gmail box and S/MIME signature for example…

So I’m trying in a last hope to post a message here to see if any of you are able to reproduce this problem at least to confort me and push it to the eM Client Support.

Thank you a lot and have a great day