Every email provider has their own quotas for how many emails can be sent in a specific period of time. As an email client, we here at Polymail are bound by these limits as well. Your email rate limit is often not static and can depend on how old your account is and whether you have been flagged for sending spam in the past. If you exceed your rate limit, your email provider can throttle your account. This can result in your email not being delivered, interruption of email sync, and other limited functionality. This is not something that can be fixed; you will need to wait until your quota resets.
Some general guidelines:
Gmail users: 500 emails per day
G Suite users: 2,000 emails per day (500 for new / trialing users)
Outlook and Office 365 users generally do not have defined rate limits. These can vary based on how many users are on your domain and how old your team is.
Office 365 Business accounts can send up to 3,600 emails per day.
IMAP - we recommend checking your documentation or contacting your provider.