Bulk email limitations explained:

There are two limits:

Standard: 500 per 10 minutes, 2500 per day.

Bulk: 2000 per 10 minutes, 10,000 per day.

Bulk is designed for users who need it for business critical purposes, e.g. Recruitment consultants emailing individuals applying for a specific job(s), support users needing to update their users and accounts departments sending invoices. 

The danger of giving all users bulk permissions is that this massively increases the risk to the company, the company’s clients and ProofPoint Essentials of a spambot (for example) causing serious IP reputation problems. 

When a spambot is detected, ProofPoint Essentials will shut down the infected address(es) to eliminate the risk of it causing any problems. Increasing the bulk limit for all users effectively removes this ability. 

There are specific third-party programs that are free to use and are designed for bulk sending, these would be recommended for services such as sending marketing material like newsletters.