To keep our systems healthy and combat spam, email providers limit the amount of mails each user can send. These limits restrict the number of messages sent per day and the number of recipients per message. So you'll encounter an error when you send emails from Freshsales using your email provider's SMTP server.
For ex, say the max limit for the number of mails is 1000 per day and you attempt sending more, an error message pops up. 

Sending limits can change without notice. Limits per day are applied over a 24-hour period, not any specific time of the day.

Each provider has a different limitation. You can find the limits for the most used email providers below:

Gmail

After reaching one of the below limits, a user can't send new messages for up to 24 hours. However, they can still receive incoming email. After this suspension period, sending limits are automatically reset and the user can resume sending mail.

These are the major limitations set by Gmail:

For more details refer Gmail sending limits in G suite

Office 365

Message rate limits determine how many messages a user can send from their Office365 account within a specified period of time. This limit helps prevent overconsumption of system resources by a single sender.

These are the major limitations set by Office 365:

For more details refer Office365 sending limits

Refer respective support articles to know the daily limits for other email providers.

Recommendations for sending bulk emails

To avoid exceeding limits while sending bulk mails, like in case of email campaigns, send to one group of recipients, wait for 24 hours and then target another group.