Gmail has certain limits in place and you only send or receive a limited number of emails per day. The daily limits are 2000 email messages for Google Apps (G Suite) accounts and 500 for consumer Gmail accounts.
It is important to know that Gmail sending limits are per user and are shared by all of the user’s email clients, including web apps, software programs, SMTP clients and add-ons that could be sending emails through your Gmail account.
For instance, if you have sent 300 emails through the Gmail website, 50 emails through Microsoft Outlook linked to your Gmail account via IMAP or POP, and another 100 messages through an email alias on a different domain, the maximum number of emails you can send through the Mail Merge addon will be just 50 in that 24 hour period.
If you exceed the sending limit, you’ll get an error message - like “you’ve reached a Gmail limit for sending email” or “Oops.. the system encountered a problem” - and Gmail may sometimes temporarily block you from sending new messages. The limits are not applied at any set time of the day and the user may have to wait up to 24 hours before the limits are reset automatically.
[*] The daily sending limit is even lower - 100 total recipients per day - if you are using Microsoft Exchange or any non-Google SMTP service for routing messages through Gmail email servers.
If you get a bounced email from email@example.com - with the message saying “An error occurred, your message was not sent” or “You have reached a limit for sending email, your message was not sent” - it indicates that you have reached the Gmail sending limit for the day.
There’s no workaround to this problem and you’ll have to wait until Gmail resets your email quota. After the quota is reset, you can resend the message to the same recipients and they should be delivered as normal.
Your messages can also bounce if you are sending too many emails to non-existent email addresses. It is thus important to clean up your mailing lists and remove invalid email addresses before running another Mail Merge campaign.
This happens when Gmail classifies your outgoing email message as spam. This happens when you have spam text in the message body or if you have a large number of recipients in the CC or BCC list.