Following the trend started by large service providers, our company has also introduced SPF-based filtering to strengthen secure mail.
According to the current state of the Internet, the sender of an email can be easily forged, and any email address can be entered as the sender (even email addresses of banks, utility companies, government offices, etc...).
In the DNS of domain names, it has long been possible to determine which IP addresses or networks are allowed to send mail on behalf of a given domain.
The SPF-based filtering method is based on checking the SPF record of the domain indicated in the email sender and taking this into account, it is highly efficient in filtering out spoofed mail.
The fact that Gmail made this method mandatory this year has played a major role in making it an industry trend.