but this regularly trips up.
How does it "trip up"?
Many ISPs limit the volume and frequency of the emails you send out. Thus, the issue may be with your ISP instead of mac mail.
Major Internet Service Providers - Email sending limits & restrictions
This article contains information about the email sending limits and restrictions of many major email service providers. The information is intended as a guide only. Your ESP can change its outbound email policy at any time without any prior notification. It is always a good idea to contact your provider and ask about the email send rate before proceeding with a mass email campaign.