Say you have a community with 100,000 users. Can an internal Salesforce user send email to all community users on the same day? According to the below link, you may do so.
I just need to confirm that because a Salesforce Support representative claims 5000 outbound emails is the limit.
https://help.salesforce.com/articleView?id=sf.limitations_email_mass.htm&type=5
Since community users are Users, if you programmatically send emails to them using their User ID this will not count against the limits. Sending via email address or using Contact IDs, on the other hand, does count against the limits.
To minimise use of the limits, even when accepting a list of email addresses, we search for users that match the email address directly or via their contact and use the User ID instead.
The 5000 limit applies only it you use apex or leveraging Salesforce API to send outbound emails
Note if you are using the Salesforce standard screens then these limits do not apply! However sending to 100000 portal users is not possible one by one using UI and you will need an API(Say you see using tools like Data loader to insert and trigger this notification). Hence these limits will apply!
I suggest you use full copy sandbox to test this limit!
Edit
If you use certain methods like setTargetId() in apex that uses ContactIds or LeadID or UserId, these limits do not apply.
However if your apex uses an email address then these limits do apply!
My suggestion if you are planning to send 100000 emails per day please use another tool that's meant for it! Like marketing cloud or AWS SES service or MailChimp and so on
