Add option to check mail for every 10 or 30 minutes

[Gamer750]

Checklist

• I have used the search function to see if someone else has already submitted the same feature request.
• I will describe the problem with as much detail as possible.
• This issue only contains a request for one single feature, not multiple (related) features.

App version

6.801

Problem you are trying to solve

I can't get any mail while the app is off

Suggested solution

Add option to check mail for every 10 or 30 minutes

Screenshots / Drawings / Technical details

No response

[cketti]

For configuring the check interval, see folder poll frequency.

For intervals smaller than 15 minutes, see #5513.