Core email logs in Salesforce Setup sometimes show recipients that start with "warmup@"

Warmup ensures the best reputation for the MTAs that serve your organization to ensure the highest email deliverability. The warmup process involves relaying email back to the MTA that originally serviced the organization or to other Salesforce MTAs that can absorb the additional load.  On the Internet rapid email volume increases cause red flags for mail/spam filters and consequently bad reputations for MTAs.  For more information on this process, search your favourite search engine for terms similar to "email warming up and MTA reputation".

Salesforce generally uses a 30 day warmup mechanism that on day one of the new load starts relaying most of any new email load to other Salesforce MTAs to reduce the impact.  This avoids a sudden volume spike in email sent from the  MTA receiving the newly added traffic.  After this the volume of email relayed to other Salesforce MTAs is gradually reduced back to zero over a 30 day period at which point the target MTA is then responsible for sending all the email, and no load balancing is taking place.  In the email logs in Setup each email that has been involved in this process will show an extra R and D email event line tracking the receive and send via the warmup MTA. For email log reference see this article.