For example the system used for postal voting or more exactly early voting in Finland is one where there is time period(around week) when voters can vote in any early voting location around country, or in such places like embassies around the world.

The process is same as regular voting. Only difference is that vote is enclosed in envelope which is enclosed in second envelope and shipped to voting precinct to be opened later and counted with rest of the votes. During the voting process identity of voter is verified.

There is some risks here, but there really isn't much to fix that. In the end if you trust enough the step between voting and shipping votes it's very decent. Anonymity is there and also voter is verified.