When using the Email Experience Manager (EXM), the dispatch of a large number of messages might fail due to the limitation of sent messages per connection. The following error can be found in log records:
MessageTaskRunner worker thread 906 07:46:10 ERROR SendEmailError: ChilkatLog: SendEmail: .... SmtpCmdResp: 452 4.5.3 Service unavailable, batch limit reached.
To resolve this issue, download and install the appropriate patch available on this page: https://github.com/SitecoreSupport/Sitecore.Support.158128.371959/releases.
Contact Sitecore Support to get a solution for other Sitecore XP versions.