Security researcher Brian Baskin says Google has hardcoded the ability to try the password infected on ZIP files. The common factor is the password ‘infected’ and the Gmail webmail service. The reason why a security firm might ask you to submit a sample in a password-encrypted ZIP file is so that no anti-virus protection between your computer and theirs intercepts the file transfer, determining it is malicious and blocks its delivery. Perhaps its time to use a different password when submitting your samples to security companies.”]
Source: https://grahamcluley.com/shouldnt-gmail-zip-files-password-infected/