The free version of Gmail is best suited for individual emails and has limitations as to the number of emails in each 24-hour period.
The daily limit sending via POP/IMAP connections (using a mail client such as Live Mail, Outlook, Thunderbird etc.) is 100 messages per 24 hours.
The daily limit if sending through a web interface at https://mail.google.com/ is 500 emails per 24 hours.
This is an absolute limit, not an entitlement. Your mail, using these limits, may or may not bounce.
New or recent accounts may have lesser limits imposed until their reputation over time is established.
If you suddenly change your mailing habits, such as sending heaps of messages when you only sent a few before, (especially with attachments) Google may think that your account has been hacked, sending spam, or just unusual activity, and lock the account for 24 hours. If you are using a mail client (Thunderbird, Outlook, Live Mail etc to send, any unsent messages will remain in your Outbox; the client will keep trying, and your account will remain locked. Clear the Outbox if this refers to your situation.
These limits apply to the free version of Gmail (account ends in “@gmail.com”); if using Google Apps for Work ($$), higher limits may apply.
Please note that each CC or BCC adds to the count, so if you have one email with ten BCC recipients and one address in the “To” field, this will count as eleven emails.
If you exceed these quotas or send any mail which Google see as unusual activity from your account (especially a new account), you will be locked out of the account for 24 hours; continual violation may result in your account being closed for violation of Google Terms of Service; and if this happens, you may be unable to recover that account under any circumstances.
The Bulk Sender’s Guidelines will also need to be considered.