See rules for this in comment below:

The following rules should be used to determine the email header:

- When an immediate email is sent with a single actor across all activities: `<displayName> has been active recently`
- When an immediate email is sent with multiple actors: `New activity is waiting for you`
- When a daily email is sent: `Here's your summary of today's activity`
- When a weekly email is sent: `Here's your summary of last week's activity`

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The mail subjects can probably follow a similar approach:

- When an immediate email is sent with a single activity: `<activitySummary>`
- When an immediate email is sent with multiple actors: `New activity is waiting for you`
- When a daily email is sent: `Today's activity activity`
- When a weekly email is sent: `Last week's activity summary`