The "full mailbox" is unlikely to be the one on your pc. It'll be your
mailbox on your ISPs mail server. This is normally caused by selecting the
option in your mail client to leave mail on the server.
If your ISP has a web mail service (most do - see their help pages), use
that to clear down the inbox on the server. Then consider changing that
setting in your mail client to stop it happening again.
The "full mailbox" is unlikely to be the one on your pc. It'll be your
mailbox on your ISPs mail server. This is normally caused by selecting the
option in your mail client to leave mail on the server.
If your ISP has a web mail service (most do - see their help pages), use
that to clear down the inbox on the server. Then consider changing that
setting in your mail client to stop it happening again.