I only realize now that the guy who left my company two months ago,n and whose e-mail account I deleted, was the only administrator on our company's Dropbox account. Despite being the manager and owner of the company, I am a single member without admin rights.
I changed my e-mail organisation, moving to Microsoft 365 and creating an Exchange account for each member of the company, to which Kevin ([Removed as per Community Guidelines]) does not belong anymore. He actually left my company BEFORE I changed the e-mail organisation, thus although I have kept the other e-mail addresses alive for all other employees and myself, the address [Removed as per Community Guidelines] doesn't exist anymore and thus you cannot even communicate with this account. I have saved the archives of all of Kevin's e-mail until he left, but it's now just a dead mailbox.