I have inherited a Marketo setup. I have noticed that we never use the Program Channel Email Send to share our monthly newsletters. Instead, our audiences (ranging from 10,000 - 25,000 contacts) are broken up into 3,000 contact lists and emailed using a Smart Campaign flow. Sometimes, instead of separate lists, the Smart Campaign flow uses Random Sample/ Send Email followed by Remove From Flow to send.
My question: Is Marketo unable to send more than 3,000 people an email without breaking? At what size audience should I build these throttles? Should I use the Program type Send Email to set up a Smart List that sends 20,000 contacts an email?
The lists aren't being broken up because of Marketo limitations.
They're presumably being broken up because of rate limits on the recipient's side.
If there aren't Wait steps between the sends, though, this makes no sense. If there are Wait steps, you should assume they were added from experience and shouldn't mess with them.