@Eugenie20Are your guests receiving emails to their proper email addresses, or are they receiving messages into their AirBnB inboxes?
It could be that you have a virus on your computer. A virus can scan your computer and find email addresses anywhere on your computer and then send spam to those email addresses. If you have saved any email addresses from guests, a virus can find them.
Make sure to scan your computer for any viruses - check for and remove any browser plugins as some are "bad" - and also change your AirBnB logins.
If you don't know how to scan and clean viruses etc, find a computer shop - they can do it for you - not expensive, but will give you the peace of mind that your computer is clean and if the problem continues, at least you can be sure it's not caused by you.