What is the maximum number of addresses that can be entered in the "To"
field when sending a message?
That will largely depend upon your ISP. Many of them cap the maximum
number of recipients at a certain point. Another consideration is that
a lot of systems automatically discard messages with more than a certain
number of recipients specified as a method for reducing spam.
I'd suggest that if you want to send an e-mail to a LOT of people that
you create one or more distribution lists and send it to the list or BCC
it to the list. That will also spare your recipients from having to sift
through a long list of e-mail addresess in the message headers.