When importing a load of messages into Gmail they have all been marked as unread. I don't want to have to go through 15000 emails page by page to mark them as read, is there any way to do this in bulk?
Search your email with
And then select them all and mark as read
You could also use this Google script which automatically marks emails in any Gmail label (inbox also) as read when they become older than N days. Google script to mark old emails as read.
Please note that it is not a one time activity but rather a continuous "check and mark as read" script.
Open the label (or, your Inbox, or Sent Mail, etc) containing the messages you'd like to select. Click the Select: All link above your messages. Click the link that says Select all [number] conversations in [current view]. Select the action you'd like to do