Created attachment 133415 [details]
Screenshot of different messages

SUMMARY
When I view a message that is signed with PGP, KMail shows all addresses that are associated with the signing key. Because the addresses are not separated by white-space, the list becomes a very long non-breaking string (except at hyphens) and causes the surrounding frame to become too wide and overflow to the right.

The addresses are already shown with commas as separator. Simply adding a space after each one should resolve this.

I’m attaching a screenshot collage of three different messages:
- an unsigned message (i.e. without the coloured frame)
- a signed message with a short frame header. The Frame is narrower than necessary, wasting considerable horizontal space on a small screen
- a signed message with a very long frame header which necessitates horizontal scrolling

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux
KDE Plasma Version: 5.20.3
KDE Frameworks Version: 5.75.0
Qt Version: 5.15.1

ADDITIONAL INFORMATION
The screen is a 10″ at 1800×1200, the KMail layout is set to three columns for optimal use of space.