hMailServer do not change the encoding of emails unless you explicitly told it to do so by using the scripting engine.
I am not trying to not believe you. But I am not being able to replicate this issue on another server.
You could try switch off SpamAssassin
I do not have SpamAssassin enabled
ALSO what barand of router do you use? Does it do any 'mail inspection' or similar...?
good point. I am using a fritzbox. But I am connecting to imap via TLS - so it should not be able to interfere. On my workstation/machine I am running Debian with no AntiVirus scanner installed.
Additionally, just to make sure its not something happening between hMailServer <> my PC (be it my client, router, ISP, etc.) I RDP'ed onto my mail server and had a look at the eml:
That means the email is stored as "quoted-printable". So that is something either happening before hMailServer or with hMailServer (be it either internal/external/whatever). Before hMailServer I have nothing installed. E.g. it is directly connected to the internet (strato.de vserver).
The server requires (START)TLS on all ports - even 25 - that should theoretically additionally avoid any scrambling happening before hMailServer