Mass email should be not the problem. Just create a LOOP (while) and sent it to whom ever you choose.
I always write 2 mails at the same time. I don't see, why it shouldn't be 10, 100, or 2000.
But imho caching cannot be a task of vlibMimeMail. This one should be handled by your mail server. Because imho there is no way for vlibMimeMail to "know", if your mail server is idle or its cpu is 100%.
You have to watch your mail server yourself or write routines (e.g. cron jobs), which does that for you.
In your LOOP you could program a time delay, though.
Best regards,
Claus