Users might experience an issue caused by the expiration of the sitecoremail.com domain that was used in the return path of the emails sent via EXM Delivery Cloud. Since the domain is no longer valid, in some cases, email providers bounce emails with such a return path.
To resolve the issue, change the value in the Sitecore.EDS.Providers.SparkPost.config file as follows:
bounce@spe.sitecoremail.com
should be replaced with:
bounce@sitecore.mail.e.sparkpost.com