To send reminders to Suppliers directly by email (instead of in-app), a default email client must be configured.
Windows: Settings (Win + I) → Apps → Default apps → "Email" section → set your preferred app as the default for Mailto links.
macOS: In your email client (e.g. Outlook) → "Mail" menu → "Settings" → "General" → select "Default email reader".
Tacto then automatically opens a pre-filled email (subject + link to the Request) in the relevant places.