This is something that has occasionally happened in the past, and has got worse since I started using Yosemite in the last month, but right now I can hardly use Spam Sieve at all as every time I mark mail as Spam Mac Mail hangs. It does come back if i’m prepared to wait several minutes, but in the last 20 minutes I’ve had to re-start it 5 or 6 times.
I find Mac Mail on Yosemite very annoying at the moment, due to general difficulties, but Spam Sieve is area I would like to get working in particular.
Also sorry to send numerous replies, that is another problem I am experiencing this evening with Mac Mail, it looks as if it is not sending mail, when it is.
I have now marked most junk mail, so may have to wait a little while, but will send the Mac Mail sample as soon as possible.