Maybe the daylight saving issue appears because smartermail uses other definitions for the beginning of standard and daylight than other clients.
For middle european time zone the daylight begins yearly at the last Sunday in March.
Smartermail users
RRULE:FREQ=YEARLY;BYDAY=5SU;BYMONTH=3 while others (e.g. outlook) use
RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=3
For middle european time zone the standard begins yearly at the last Sunday in October.
Smartermail users
RRULE:FREQ=YEARLY;BYDAY=5SU;BYMONTH=10 while others use
RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
Here the relevant part of the VCALENDAR text:
BEGIN:VTIMEZONE
TZID:Europe/Berlin
X-LIC-LOCATION:Europe/Berlin
BEGIN:STANDARD
DTSTART:00011104T030000
RRULE:FREQ=YEARLY;BYDAY=5SU;BYMONTH=10
TZNAME:Mitteleuropäische Zeit
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:00010405T020000
RRULE:FREQ=YEARLY;BYDAY=5SU;BYMONTH=3
TZNAME:Mitteleuropäische Sommerzeit
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
END:DAYLIGHT
END:VTIMEZONE
Hope, this helps.