You can send single emails to a maximum of 5,000 external email addresses per day through Apex or API.
Email sent through the send email button don’t count toward this limit, like sending email from Account, Contact, Lead, Opportunity, Case, Campaign or custom object pages.
You can send an unlimited amount of email to your org’s internal users, which includes portal users.
Single Email –
You can specify up to 100 recipients for the To field and up to 25 recipients for the CC and BCC fields.
If you send email to your org’s internal users, specifying the user’s ID in setTargetObjectId then email doesn’t count toward the daily limit.
If any organizations hit their SingleEmailMessage limit then they will receive the error such as SINGLE_EMAIL_LIMIT_EXCEEDED, when sending the email through API or Apex.
Mass Email –
You can send mass email to a maximum of 5,000 external email addresses per day per org based on GMT.
You can send mass emails only to contacts, person accounts, leads, and your org’s internal users.
If you are running the Professional Edition of Salesforce, that’s 250 emails in a single call, or if you are running Enterprise Edition, 500. For Unlimited Edition, you can send 1,000 emails in a single call. The native mass email function is only available in Professional, Enterprise, Unlimited, and Developer Editions. So, if you have 4,000 emails that you need to send and you’re running Professional Edition, you’ll have to break up your email list into 16 call of 250 emails each, and schedule them to be sent out in 4 call of 250 emails a day for 4 days.
Through Apex/API (MassEmailMessage class) only you can send email to contacts, leads, or users. In a single method call you can send email to 250 email addresses in Professional Edition.
But currently you can’t track in salesforce setup. However you can track the limits and usage for the organization using the Workbench tool.
1. Go to https://workbench.developerforce.com/login.php and provide your salesforce credentials.
2. Accept oauth prompts and then select “Jump to REST Explorer” and click on execute
3. From the options presented select: /services/data/vXX.0/limits
4. Select the SingleEmail area to view the daily maximum and remaining calls.