POP3: mails are fetched on account setup only

Context

Expected Behavior

Dekko should fetch new mails.

Actual Behavior

Dekko fails on fetching new mails using POP3.

Steps to Reproduce

1. Setup an account using POP3 (mails are fetched successfully)
2. Sent yourself a new mail and wait until mail should acutally have been fetched

Your Environment

• Dekko version used:
• Desktop environment (e.g Unity7, Unity8, GNOME):
• Operating System and version: