KOrganizer5 is being used from within the Kontact suite. There are three e-Mail identities, three POP-3 mail-boxes and, three SMTP transports defined. When an appointment is sent as an iCal to one or more e-Mail recipients, despite the default SMTP transport being configured in KOrganizer and, the same default SMTP transport is defined in KMail2 and, '~/.config/mailtransports' also points to the correct default SMTP transport, KOrganizer uses another SMTP transport but, includes the correct default user's e-Mail address. KOrganizer is configured to use the "default" KMail2 identity. Changing the alphabetical sorting of the SMTP transports by prepending "ZZ-" to the SMTP transport names which are not the "default" SMTP transport, doesn't help. Looking at the contents of '~/.config/mailtransports', the SMTP transport which KOrganizer always uses is the 2nd transport ID in the list.
With the following KDE components (openSUSE Leap 15.0) this issue seems to have been resolved: korganizer 5.7.3 KDE: 5.45.0 plasmashell 5.12.5 Qt: 5.9.4 Now closing this issue.