An SMTP (Simple Mail Transfer Protocol) server is commonly used for sending bulk emails due to several reasons:

Reliable email delivery:

 SMTP servers are designed to handle the task of sending emails efficiently and reliably. They have built-in mechanisms to handle large volumes of emails and ensure their proper delivery to the intended recipients

Authentication and security:

SMTP servers often require authentication to prevent unauthorized use. By authenticating with the server, you prove your identity and reduce the chances of your emails being flagged as spam or rejected by recipient servers. Additionally, SMTP servers can implement security measures such as encryption to protect the confidentiality of your email content.

IP reputation management:

Sending a large number of emails from a single IP address can potentially affect your IP reputation. SMTP servers typically have mechanisms in place to manage IP reputation and maintain good sender reputation scores. This helps ensure that your emails are more likely to reach the recipients' inboxes instead of being filtered as spam.

Throttling and delivery control: 

Bulk email campaigns often require control over the rate at which emails are sent to avoid overwhelming the server or triggering spam filters. SMTP servers provide features such as throttling, which allow you to control the number of emails sent per unit of time. This helps maintain a steady and controlled flow of emails, improving deliverability.

Tracking and analytics:

SMTP servers often provide tracking and analytics features that allow you to monitor the performance of your bulk email campaigns. You can gather data on email opens, clicks, bounces, and other metrics that help you evaluate the effectiveness of your campaigns and make necessary adjustments.

It's worth mentioning that while it's technically possible to send bulk emails without an SMTP server, using a dedicated SMTP server or a reputable email service provider ensures better deliverability, scalability, and compliance with email sending standards and best practices.