Mandatory if templateId is not passed. Pass name (optional) and email or id of sender from which emails will be sent. name will be ignored if passed along with sender id. For example,
{"name":"Mary from MyShop", "email":"no-reply@myshop.com"}
{"id":2}

Mandatory if messageVersions are not passed, ignored if messageVersions are passed
List of email addresses and names (optional) of the recipients. For example,
[{"name":"Jimmy", "email":"jimmy98@example.com"}, {"name":"Joe", "email":"joe@example.com"}]

List of email addresses and names (optional) of the recipients in bcc

List of email addresses and names (optional) of the recipients in cc

Email (required), along with name (optional), on which transactional mail recipients will be able to reply back. For example,
{"email":"ann6533@example.com", "name":"Ann"}

Pass the absolute URL (no local file) or the base64 content of the attachment along with the attachment name. Mandatory if attachment content is passed. For example,
[{"url":"https://attachment.domain.com/myAttachmentFromUrl.jpg", "name":"myAttachmentFromUrl.jpg"}, {"content":"base64 example content", "name":"myAttachmentFromBase64.jpg"}].
Allowed extensions for attachment file:

xlsx, xls, ods, docx, docm, doc, csv, pdf, txt, gif, jpg, jpeg, png, tif, tiff, rtf, bmp, cgm, css, shtml, html, htm, zip, xml, ppt, pptx, tar, ez, ics, mobi, msg, pub, eps, odt, mp3, m4a, m4v, wma, ogg, flac, wav, aif, aifc, aiff, mp4, mov, avi, mkv, mpeg, mpg, wmv, pkpass and xlsm.

If templateId is passed and is in New Template Language format then both attachment url and content are accepted. If template is in Old template Language format, then attachment is ignored

Pass the set of custom headers (not the standard headers) that shall be sent along the mail headers in the original email. 'sender.ip' header can be set (only for dedicated ip users) to mention the IP to be used for sending transactional emails. Headers are allowed in This-Case-Only (i.e. words separated by hyphen with first letter of each word in capital letter), they will be converted to such case styling if not in this format in the request payload. For example,
{"sender.ip":"1.2.3.4", "X-Mailin-custom":"some_custom_header", "idempotencyKey":"abc-123"}.

Pass the set of attributes to customize the template. For example, {"FNAME":"Joe", "LNAME":"Doe"}. It's considered only if template is in New Template Language format.

You can customize and send out multiple versions of a mail. templateId can be customized only if global parameter contains templateId. htmlContent and textContent can be customized only if any of the two, htmlContent or textContent, is present in global parameters. Some global parameters such as to(mandatory), bcc, cc, replyTo, subject can also be customized specific to each version.
Total number of recipients in one API request must not exceed 2000. However, you can still pass upto 99 recipients maximum in one message version.
The size of individual params in all the messageVersions shall not exceed 100 KB limit and that of cumulative params shall not exceed 1000 KB.
You can follow this step-by-step guide on how to use messageVersions to batch send emails - https://developers.brevo.com/docs/batch-send-transactional-emails

Tag your emails to find them more easily