Your comment assumes knowledge of orage. Please provide more info.

I gave myself a quick crash course in Orage:

I exported a calendar in Claws Mail to an .ics file.

In Orage I opened /File/Exchange data.

Then use 'Add new foreign file' to select the exported .ics file, and clicked 'Add'.

Then the meeting in the exported .ics file was visible from Orage's calendar window.

[Sure, what do you need?]
The vCalendar plugin has a setting to register the Claws calendar in XFCE Orage. With activating the setting, I expected to either see the dates from Claws in Orage directly (somehow), or to see the Claws calendar in Orage in the 'Foreign files' tab in the 'Exchange Data Window' (https://docs.xfce.org/apps/orage/application_windows#exchange_data_window), where Orage would then show the dates from that Claws calendar, as soon as they get changed/added/deleted.

Both things do not happen. Orage does not show the dates from Claws, like the plugin's setting suggests it would do. 
(Still one could add the Claws internal.ics manually as foreign file to Orage, but then the Claws setting for it would not be needed.)


Probably some DBus call from the Claws plugin to Orage could solve the issue, but I am not that much into Orage to be a real help for that, although at least there seems to be a method AddForeign, which sounds very much like it could do the job. (LoadFile will do a once-only import of dates from a .ics file into the default Orage calendar, without reflecting any further change of that .ics)

```
$ qdbus org.xfce.orage /org/xfce/orage                                                                      method QString org.freedesktop.DBus.Introspectable.Introspect()
method QDBusVariant org.freedesktop.DBus.Properties.Get(QString interface, QString propname)
method QVariantMap org.freedesktop.DBus.Properties.GetAll(QString interface)
method void org.freedesktop.DBus.Properties.Set(QString interface, QString propname, QDBusVariant value)
method void org.xfce.orage.AddForeign(QString file, bool mode, QString name)
method void org.xfce.orage.ExportFile(QString file, int type, QString uids)
method void org.xfce.orage.LoadFile(QString file)
method void org.xfce.orage.RemoveForeign(QString file)
```

On further testing, the option "Register Claws' calendar in XFCE's Orage clock" also works for me: a meerting created in Claws Mail instantly shows in Orage when the meeting day is selected in Orage.

DBus is used. If Claws Mail is not built with DBus support then the option "Register Claws' calendar in XFCE's Orage clock" is enabled. Otherwise it is greyed-out and inactive.

Like I hate these things, but works for me too … today.
No idea why, had many combinations tested yesterday, and not even a reboot in between…

At least of course nice it works :-)

Nice. Thanks for reporting back.