I too have this issue and Avast is definitely the problem which came with Version 7. I have a spam filter at ISP level so don't need the Avast filter. Until Avast fixes things you an solve by turning off the Avast antispam filter as follows-
1) Open the Avast control panel by clicking on the Avast icon in the task bar (bottom right of the desktop)
2) On the left of the control panel select ADDITIONAL PROTECTION then 'Antispam'
3) The next window will show at the top PROTECTION STATUS: RUNNING (in green). To the right of that is a 'Stop' button. Click on the 'Stop' button and it will then ask you if you want to stop for 10 mins, 1 hr, until restart of computer or permanently. Choose the option, confirm and that's it.
I don't mind having a second level of spam filtering, but Avast should fix the problem asap. Until then it will be 'off' on my computer.
Hope that helps.