@Don_Armstrong -- have you tried splitting your 600+ members into 7 "groups", on your computer. Then, send your message to one group at a time.
Or, have you considered "out-sourcing" your mailing-list to a commercial company,
e.g., LISTSERV (tm): https://www.lsoft.com/resources/learnlistserv.asp
which is compatible with "high-volume" communications?