get rid of hyphen in digest emails

I've a problem with Messaging/Notifications. As I don't know where the problem can be in these two modules I posted this request also in Messaging issues.

Not sure if this is one or the other module but I have some hyphens under each title and I need to get rid of this.
I found for the "--" footer separation but not for the others.

Is there somone that could help for this? Many thanks in advance

François