I want to keep my Sent Mail folder (label would be more correct) clean, i.e. move done messages to some archive and keep only a few that remind me of something. In the inbox I can do this by archiving an email. It is gone from the inbox but not deleted altogether. In the Sent Mail folder this doesn't work: when I archive a sent email the Sent Mail folder keeps listing it.
Actually, Sent mail in Gmail isn't a folder nor a label. It's a system view similar to "All Mail".
The only way to remove messages from there is by deleting them.
As @user829755 found some web pages suggests the use of IMAP client to remove messages from the
Sent. According to bkc56 in Managing Sent Mail this could work temporarily but once Gmail refreshes the user mailbox index, the removed messages will appear in
How can I clear Sent Folder. I have assigned emails to Labels. If I delete them out of Sent Mail it will delete all. - Gmail Help Forum
Rubén's answer sounds reasonable but then I found this URL which describes how to solve the problem via the imap interface with external email client. I tried it out and it seems to work. I have now emails that I wrote myself and they appear in All Mail and under some label I gave to them but no longer in the Sent Mail folder.
But: see Rubén's comment
Here is a creative solution I have just started using. I created labels sent2016, sent2015, sent2014, etc. Then I applied the labels, for example search for sent emails
before:2016/01/01 after:2014/12/31, select all, then click on "Select all messages that match this search", then apply the label "sent2015". After doing this for each completed year, I then labeled all the current year's sent messages too (sent2016).
The next step, for me, since I read my email in Thunderbird, was to unsubscribe the Sent folder, and to subscribe to sent2016 instead. Now when I look at messages in sent2016, I'm only looking at the current year, not 8 years worth of messages!
I think this would also be somewhat helpful if I were reading my gmail in the browser, too, because instead of checking the "Sent" box, I could look in the "sent2016" label.
(I did select all the old sent messages labeled "sent2015", for example, and hit "archive"; but I don't know that that was particularly helpful....)
P.S. I also unchecked IMAP for the old labels.