1) Your email has been detected as spam
This is the most common case. Google may detect your emails as spam. You may need to review your email campaign so as to prevent your email to fall into spam. Check our section How to avoid spam.
2) Your G Suite account is new and under trial
If you are a new G Suite user, your account may be under trial and limited to 400 emails per day. Usually, the trial period lasts two weeks. You can skip the G Suite trial period by making a manual payment or pay early.
3) Your daily sending limit has been hit
Your daily quota includes mails sent with Mailmeteor but also emails that you manually send with Gmail. You may have sent some emails previously with Gmail? If so, try again in a few hours. Read more on Gmail's sending limits.
4) You are sending emails using POP/IMAP
We recommend you to remove this setting if you want to use Mailmeteor properly without limit.
If you still need a helping hand on this, contact us. We'll be glad to help!