I suggest you pursue a mail manager app, such as Mailmerge, which is free for small lists. On your question, you could divide the list to four of 100 each and use send later then. but you expose your account to being flagged potentially as a spam generator.

Your correct, using Send Later divided up and sending throug BCC we not ony wound up as a Spammer, but also wound up on a few Black lists. I'll bring up the MailMerge app at our next board meeting. Thanks.. We are a small local Non Profit ComalDarkSky.--g

I encourage you to not rely on an email account for your organization's mailing. That should be done by a specialized company. Mailchimp offers discounts for non-profits, and Mailerlite is free for up to 500 email addresses. These or others are you best, and probably only, means to avoid being seen as spammers. Further, mailing organizations often provide other services. For example, I administered a mailing list or a town government and, after using Mailchimp for a few months, I discovered a number of reports, one of which showed that only 50% of recipients were opening the mail. Good luck to you.