Documentation
Step-by-step guides. There is very little of MailWhen that needs documenting — the address is the instruction, and the addresses page is the whole vocabulary. What does need it is getting those addresses into an address book, because every address book wants a different file and two of them fail quietly.
Adding the addresses to your contacts
Google Contacts
Four steps, on a computer. For Gmail and Google Workspace, plus the one iPhone setting that decides whether they show up at all.
Microsoft Outlook
Six steps. Includes unblocking the file, which Outlook requires and reports as a permissions error, and why the vCard imports exactly one contact.
On an iPhone or iPad none of this is needed: open mailwhen.vcf on the phone and choose Add All Contacts. All three files are on the Contacts page.