A base58-encoded extension part, for segmenting actions due to email (and the replies/responses to/consequences of email) originated by a SaaS platform. Helps with routing of support requests in particular so we don't have to go back to the end-user to say "which <platform organisation> did you mean?" and other similar CRM-ish behaviours. Also allows us to manage bounces, spam complaints, and RTBF assertions by (organisation,end-user) tuple and similar. The discriminator string itself comes from an application subsystem where it was already generated for our state machines. The (minor) downside is outbound delivery sometimes being delayed by greylisting more frequently than otherwise.

Since I've contributed to MTAs & MDAs and built multinational ISP email services in a previous life, I'm confident that every MTA of consequence is case-preserving of envelopes, I'm happy to rely on it. (this is also why I feel on solid ground pointing out the hidden gotchas in various proposed schemes that don't perfectly accommodate the same rule)