The Notes native client has an option to download all attachments to a folder. I cannot found similiar on web based iNotes (8.5). Is there a way to download all attachments at once ?
You can select multiple emails and select Forward, this will combine all attachments into one window. After that click on one of the attachments and in contextual menu select Save All.... However it may not work quiet well when the files have the same filename, but it depends on the operating system whether it can rename the same files.
When you're using specific mail provider, another way is to login to the web mail (e.g. Office 365) and download from there. If no option is provided, use some browser plugin to repeat the same action over and over again (like Selenium IDE or TamperMonkey).
Or forward group of e-mails with attachments to another address which is easier to access (e.g. gmail can group the attachment into single zip file, even when they've the same file names).
Another way is to open the inbox file format directly (NSF format) using alternative software (optionally you can select and archive bunch of e-mails into some specific folder, e.g. in Actions you can Archive Selected Documents into the file). On Windows, the mail files can be found in \Program Files\IBM\Lotus\Notes\Data\mail. See: How do I find where a local Lotus Notes database is?