I have noticed that threads with a long title cause problems when replying by e-mail.
It seems that spaces are removed from the subject which causes a new thread to be created.
Example:
I received a mail with subject:
[SlimDevices: Plugins] Custom IR maps - available function/menuitems list

I replied with a mail with subject:
RE: [SlimDevices: Plugins] Custom IR maps - available function/menuitems list

However what was returned to me by email & posted on the web site had the following subject:
RE: [SlimDevices: Plugins] Custom IR maps - availablefunction/menuitems list

so you see that the space between available & function has been removed,
causing two separate threads to be created, both in the web interface and the mail client. 
When the title is short, this doesn't seem to happen.

I am replying using Outlook on WinXP if that's an issue, but it seems it's happening in the mail gateway?