Exchange: How to reliably determine that a user is logged into mailbox?

Sometimes (for example while migration users are from another email system) there is a need for reliable determination that a user is logged into the mailbox (using Outlook or OWA).

The simplest answer is mailbox's lastlogontime (Get-MailboxStatistics). But it turnes out to be not quite true.
Attention! The attribute "LastLogonTime" of user's mailbox filled and updated when another user opens user's calendar.

Finally, I found 4 criterias which help to reliably determine that a user is logged into the mailbox.

1. "lastlogontime" (Get-MailboxStatistics) - necessary but not sufficient attribute.
2. "Languages" (Get-Mailbox) - this attribute filled when user entered into OWA and sometimes filled when entered into Outlook.
3. "WorkingHoursTimeZone" (Get-MailboxCalendarConfiguration) - this attribute filled when user entered into OWA and sometimes filled when entered into Outlook. Deffault value - "Pacific Standard Time", so you can use it only if users are in a different time zone.
4. "Quick Step Settings" (Get-MailboxFolderStatistics) - this is not an attribute, it is a checking for the presence of a folder "Quick Step Settings" in the mailbox, because it appears only when you open/configure Outlook.

Exchange 2016: How to move system mailboxes

Exchange 2016 has 5 types of system mailboxes: Discovery Search (1 mbx), Arbitration (6 mbx), AuditLog (1 mbx), Public Folder (not created by default) and Monitoring (the number depends on the number of databases). All of them are necessary for full-fledged Exchange's work.

You can't delete the first mailbox database (or another one) until all system mailboxes migrate to a new database.

Exchange 2016 on Windows Server 2012 R2 Core

I know that Microsoft doesn’t support Exchange 2016 on Windows Server Core or Nano Server. But I decided to find out is it possible or not.

During install Exchange 2016 CU3 on Windows Server 2012 R2 Core (Standard Edition without "Server-Gui-Shell" and "Server-Gui-Mgmt-Infra" features) with checked "Automatically install Windows Server roles and features that are required to install Exchange Server" wizard installs GUI by self.