We are using CiviCRM 5.35.0, but I have tracked this issue down to the installation of 5.34.0 (updated from 5.33.2). Our CMS is Drupal 7.78. Our PHP is 7.3.27. Our outbound mail setting in CiviCRM is "mail()" and this continues to work for most mailings.
However, starting right after I installed 5.34, when a contribution is made, CiviCRM does not seem to actually send the expected emails (receipt AND profile submission notifications). In the contact's contribution record, it says that the email receipt was sent, but neither the contact nor myself (I am cc'd on those receipts - setting on the contribution page) gets an email.
Our Payment processor is Stripe and we are using the Stripe Payment Processor extension version 6.5.7. All extensions are up to date. Stripe continues to send its receipts directly, so at least our users are getting those.
As far as I know, this issue is only occurring when emails are triggered by a contribution. Otherwise, the system's emails seem to be going out as expected.
Anybody have any ideas what might be causing this?