To recreate this bug: - go to administration-> SMTP servers and click new smtp server - fill in the required info with random inputs, just make sure to select the box that says default server. Click submit when finished. - now click on the edit button next to the server you just created. Notice that the 'Default server' box is still selected. Press submit. - notice how even though the box was selected, the server is now not the default server. - click on edit again. - there is a weird blue box in the background of the form - select 'Default server' and submit - the server is now default as expected.
I tested and it seems to work OK now, either fixed by bug 34622 or bug 34616 I reckon.