I am a registered user of MailWasher Pro, but I am getting messages that my trial period is running out.

You need to copy and paste your registration key into the Register form by clicking the Register button from the main button menu in MailWasher Pro.

If you no longer have your registration key, you can request it to be emailed to you through MailWasher Pro. Just click Register on the main button bar and enter your email address in the I have registered but lost my key field.