I'd like to know how I can mark all messages with a certain tag as read, all at once.
I have around 2000 unread messages with a certain tag, and I don't want to go through endless pages of "select all" -> "mark as read"
There are two ways:
You can also do this with a filter, by creating the filter, choosing to mark the messages as read, and then choosing to apply the filter to the matching conversations. Then simply delete the filter.
Click 'All' next to select. It selects first page of messages Click 'All N messages' after new prompt is shown and confirm Click 'Mark as read'