What is the ruling on sending an email to female employees containing a benefit or a religious ruling? Does this open a door that is difficult to close? And should a joint email be sent to a male and female employee discussing a non-work-related topic that includes a religious ruling for this mistake?

Sending scholarly messages and good admonitions to women is permissible in principle. However, experience has shown that opening this door can be a strong pretext for fitna (temptation/strife) and evil. Therefore, every door and pretext that leads to that should be closed.

If you desire widespread benefit, you can task your wife or sister with communicating with women and inviting them (to Islam or good deeds).

As for women conversing with men without necessity, it is not permissible.

If you encounter an evil ( munkar ), you must immediately forbid it with wisdom and good admonition. Delaying the prohibition is not permissible, due to the Almighty's saying: "And when a community among them said, 'Why do you advise a people whom Allah is to destroy or to punish with a severe punishment?' They said, 'As an excuse to your Lord and perhaps they may fear Him.'" (Al-A'raf: 164).

If you believe that direct prohibition will lead to a greater harm, then there is no objection to denouncing the evil in your heart and delaying the reminder to another time.