Message bounced / not sent due to sending limit
Why have my emails not been sent out?
When you complete a mail merge, you might notice that some of your emails have not been sent out. Yet Another Mail Merge specifies it in your spreadsheet with the mention 'EMAIL_NOT_SENT' on the 'Merge status' column:
This happens because we failed to calculate properly your remaining quota. For some reasons, YAMM detected that you had fewer quota than it actually was.
To make sure that your recipients receive your email, you will need to resend it to these people. Check that you have enough quota within a 24 hours timeframe to avoid any blocked account (see section below).
Why did Google block my Gmail / Google Apps account?
In an effort to fight spam, prevent abuse and unusual activity, Google will temporarily disable your account if you send messages to more than 500 recipients or if you send a large number of undeliverable messages.
That's why the hard cap on emails you can send (even with a paid plan) is 400 emails per day (unless you have a Google Apps account). So if you send all the emails you can through YAMM, you'll still be able to manually send 100 emails from your Gmail account with no issue.
Note that we do not have any control on the access to your email address. If your Gmail or Google Apps account has been blocked, please contact Google directly.
More information: https://support.google.com/mail/answer/43692?hl=en
How to avoid your account to be blocked again?
If, at some point, you see the error message 'You have reached a limit for sending mail. Your message was not sent.' in Gmail directly, it means for some reason your account has a smaller quota than standard Gmail accounts.
Possible causes are:
You sent a large number of undeliverable messages (messages that bounced back)
You have linked a POP or IMAP client (e.g.: Microsoft Outlook or Apple Mail) with your Gmail account. You may only send a message to 100 people at a time
You have a reply-to address from a different domain
You are sending many emails with another tool
You are leaving multiple instances of Gmail open
Therefore, you will have to check if one of the cases cited above apply to you, and fix this issue on your end.
More information: https://support.google.com/mail/answer/22839