Mailwasher only shows a copy of the mail on the server when it checks for new email. Then it downloads a portion of the message so you can then see if it is spam and decide to delete, or keep it. But emails always remain on the server until they are downloaded by your email program.
Most likely you have your email program setup to leave a copy of the message on the server, or you just need to check mail again in MailWasher to clear the screen.
If using POP3 to get your emails, then you can turn off this option to 'leave emails on the server' in your email program. See here for a list of email programs with instructions on how to do this:
Or, another possibility is that this account is setup as IMAP as opposed to POP. Either change the account type to POP in your email client, (you may need to delete the account first, then set it up as POP), OR, perhaps easier, you can change to IMAP within MailWasher under Settings>>Accounts, click your email account, click the Incoming tab, and beside "Server Type:"
select IMAP in place of POP.
Click save to save changes.
Then, in MailWasher click, Settings>>Accounts, click your email account, and click the Incoming tab>>Advanced, and ensure that, "Download read emails" is not ticked there.
This way, MailWasher won't show all your old emails that are left on the mail server.