I got the email for your forum post and it is not truncated to 33 characters:
"[mojoPortal - Feature Requests] Subject header of a email when the email is sent from a forum by means of DotNetOpenMail"
So I don't think its a problem with DotNetOpenMail. There is a message template file used for the subject and another one for the body. In ForumPostEdit.aspx.cs you can see this code:
Default culture is going to be from the Web.config setting:
default in this case is en-US so the template file will be /Data/MessageTemplates/en-US-ForumNotificationEmailSubject.config
if you have changed the default then maybe there is no template existing for the default culture and this could cause the problem you are seeing. The subject is formatted like this:
So most likely you are using a different culture and need to create a message template for your chosen culture.
Hope it helps,