"Send failed: Invalid property or value." is Stalwart's description for invalidProperties, and on its own it says nothing about what to fix. The SetError also carries a `properties` array naming the offending fields, which every call site was discarding. setErrorMessage appends them, and the 35 places that surfaced a SetError - send, save draft, mailboxes, calendars, contacts, sieve, files, signature images, sharing - now go through it.