Quarantine feature

[brinza888]

In #3169 I asked about ability to store rejected emails and be able to view them. This usually called quarantine feature. And I think many Mailu users will be happy to see it.

These reasons are why this feature may be interesting for others:

• Much easier for admin to understand why email was rejected
• The sender will not need to resend the email, so admin will not need to ask him about this
• Admin will not need to weaken spam filtering options only to receive few important emails

Thank you in advance!