Why do I need to do this in batches?
I don't want to do anything as BCC as each message will require a personal Dear XXXX.
Can I do batches of 50 at a time all using just the to: field?
Ah, I missed the personalization part, my apologies. Yeah, you'll need to send one at a time, if the greeting is crucial to the mailing. Batches are used (when mass mailing the same subject/body to different recipients) to reduce overhead, as sending one email at a time is very resource intensive, i.e. memory, bandwidth, the MTA i.e. sendmail, mail queue, DNS hostname lookup, log updating and so on. There is typically a delay between batches so the server can recover those resources for the next batch. And if the server is used for other things, i.e. web or apps, scheduling the mail to be sent when things are slow is wise.