To help prevent spam and keep accounts safe, email accounts limit the number of emails you can send every day.
By default, the limit for each email account in Remail is 400 emails per day. If you use G-suite account you can increase the limit for 800 emails per day; if you use your corporate server you can add settings to your server (but in this case you are solely responsible for the potential blocking by spam filters).
First of all, sending emails through SMTP relay services (like with Remail) should look like they are written personally for each recipient and send manually. So these limits help you to keep human-like actions for spam filters and receiving servers.