If you use pop3 as an e-mail protocol, and have settings on all clients set to 'leave on server', this is the result.

Better switch to imap (if your email provider and email programs/apps allow) or to full webmail in stead of local clients with local storage. That was quite useful when (long ago) connection speed was low and you only had one device. you used for e-mail. But it's getting impractical for those - like you - that use more that one device for their e-mail.