In preparation for MX upgrades I'd like to ensure that Jenkins will not be negatively affected by mx1001 downtime.
What does the current outbound smtp config look like in Jenkins?
If smarthost redundancy is not already in place we have a straightforward (and imo preferred) option to use the localhost exim listener as the outbound smtp server. This listener was deployed to provide local queueing and failover between mx1001 and mx2001 for services that do not support this type of failover natively. FWIW many services (Gerrit, Phabricator, etc.) are configured to relay mail in this way.