Swiftmailer inserts unprintable characters when non-ASCII characters are present
When there are non-ASCII characters in the subject (in our case "Umlaute" - äöü - or french characters éàè etc.), the swiftmailer inserts unprintable characters and breaks the UTF-8 encoding for some reason.
The TYPO3-core where we had and solved the problem was always a 4.5.2 so far. But since I did not see any changes to the Swiftmailer class mentioned below, I guess it still persists in 4.7.
Stellenausschreibung - =?utf-8?Q?Ober=C3=A4rztin/Oberarzt?= sowie =?utf-8?Q?Assistenz=C3=A4rztin/Assisten?=
Should look like:
Stellenausschreibung - Oberärztin/Oberarzt sowie Assistenzärztin/Assistenzarzt
As it has been a little while since I actually debugged the issue, I do not remember where exactly this issue comes from, but it can be resolved with patching the file:
The idea is to remove all nonprintables from the subject as shown in the attached patch.
I understand this is not an ideal solution, however it was the only one I found at the time.