I a writing a Content Management System in PHP and working on a profile update reminder script that will send an email and allow users to click a link in the email to visit their profile and update their info and preferences.

This is for a closed membership group of 200-400 members and I expect emails to go out to individuals all at on time when admin chooses.

I am almost finished with this module and have HTML form letter templates working where each members info gets filled in and they get an individual URL with key in the email. ( also have opt-out links and options setup to be compliant with my host policies.)

My question is:
Is it better to send out the emails in batches of 10-20 using AJAX or just spool all 200-400 using the PHP mail() command in succession all at one time.

I do not have much server experience so I am trying to avoid pitfalls here. If it were just a newsletter I would be sending 200-400 copies of same email but each of these will contain some unique content.