Describe the bug

sender/sender.go:802-828 writes the text/plain, text/calendar, and .ics attachment parts via direct .Write calls on the multipart writers, but the returned errors are discarded:

qp := quotedprintable.NewWriter(plainPart)
fmt.Fprint(qp, plainBody)
qp.Close()
...
calPart.Write([]byte(clib.WrapBase64(...)))
...
attachPart.Write([]byte(clib.WrapBase64(...)))

A short write or buffer error here produces a malformed iMIP reply that Google Calendar / Outlook silently reject without RSVP being recorded.

Expected behavior

Capture the errors from each Write/Close and return early if any fail.