Sorry to hear about the issue with your email. There are 2 common causes that may explain why this is happening.
The first would be caused by email spoofing. Email spoofing is not virus related as it is a method to where another person sends emails from their system but is able to mask their domain name with yours which is intended to fool others in attempts to phish for information and other valuables.
The second cause would be an email compromise. This may be virus related as it means that the password to the email has been accessed and someone is using your email to send messages on their behalf, also for phishing attempts.
How to determine the cause: If you are using workspace email our system has an outgoing relay limit. relays are used when ever and email is sent from a email client. Most commonly the relays are limited to 250 a day. If you are noticing that your relays are at a high number and jumping this would be a red flag that your email may have been compromised and the best actions to take are to access your email from a secure device and reset the password. Once you have reset your password you may need to run virus scanners and malware detectors on your devices to help find the cause. Below I have provided a guide of how to check your email relays if you are using workspace email.
If your relays are not maxing out and do not seem unusually high, your email domain may be getting spoofed. To combat spoofing would require an SPF record added to your domains DNS. SPF records send a request to the receiving server to validate the domain before accepting incoming emails from that domain. Not all email servers will honor the request however most larger email servers will. Below I have included an article with more information about SPF records.