SMTP limits are set by your web or email host. Check with them to see if you're rate-limited. Most of the major hosts are, so unless you run your own server, it's likely there's some limit. At that point, limiting the sending rate would be valuable to ensure messages aren't discarded or bounced.
If you need to send all of the invitations quickly, subscribing to a cheap SMTP service or using a free mail account with a greater/no limit would be the way to go.
It might also be worth checking your web host's PHP mail/sendmail limit. It's often different to the SMTP limit, though it can be better or worse.
