On Drupal 7 + CiviCRM 5.35.2, contribution receipts are not being sent out, in spite of the claim on the contribution thank you page that they've been sent.
Instead, in the civicrm error log, I'm seeing errors saying "Could not find valid value for u", just after the messages confirming that the contribution has been made.
I tracked the error message in the civi logs and see that they started immediately after the civicrm security update to 5.35.2.
Of note is that this site uses CiviSMTP for civimails, though since these are transactional emails, it wouldn't use this (right?). More interestingly, enabling the (brilliant) extension "Transactional Mail - Bounce and Tracking Handler" resolves this issue.
All this leaves me thoroughly confused and a little worried about when CiviCRM claims a receipt has been sent.