A dynamic relay makes real-time decisions about routing to mail transfer agents (MTAs) of email envelopes received by the relay from an SMTP email server. Those decisions can be based on one or more factors, such as MTA respective statuses. They can also be based on information contained in x-header fields provided in the email messages' respective headers (e.g., identifying the individual/entity (or “client”) for whom the email envelope is being generated and/or an IP address of the MTA to select for load balancing or otherwise). This allows, for example, the relay to select among available MTAs for delivery of each message in order to segregate email message traffic for load-balancing and/or based on preferences of specific MTAs for specific clients, or otherwise, while bypassing MTAs that have been blacklisted or are suffering delays.