Note: we use 0777 for permissions because this is what works on most hosting accounts. The data that you create in the web interface has to also be available in the command line as well and vice-versa, and often the user/group under which the web process runs is different from the one used in command line, which might lead to situations where the files are created from the web interface but they cannot be removed/modified from cli (i.e: when doing cleanups/upgrades/etc) or vice-versa, like cache built in the command line but unreadable in the web interface.
If your hosting account is configured to use same user/group for the web and for the cli, then by all means try to use 0775 or 0755 for file permissions, otherwise stick to 0777.
Following folders have to be writable by the web server, that is is chmod-ed to 0777 recursive:
/apps/common/config /apps/common/runtime /backend/assets/cache /customer/assets/cache /frontend/assets/cache /frontend/assets/files /frontend/assets/gallery /apps/extensions
Mailwizz also comes with a command line script that will chmod all the above folders automatically. You just have to run it from command line like:
# make it executable chmod +x /apps/console/commands/shell/set-dir-perms # run it /apps/console/commands/shell/set-dir-perms