My T hasn't given me a clear policy on it which sometimes makes me feel anxious. My fault too, as I COULD ask.

He doesn't always reply, but he said he "welcomes" emails in which I say how I feel. I always try to keep them as short as possible.

I personally don't email him when I know he is on leave unless it's something to do with an upcoming appointment confusion. He always replies promptly to those.

However, I think that if you feel like emailing, just go for it. It's not like you're giving your T a phone call. He/She can choose whether they want to reply or not. One thing I did learn when emailing T....sometimes they won't reply. They'll wait for the next session to go through it, so as long as you're ok with that, then go ahead. I tend to get anxious when he doesn't reply to emails about how I'm feeling, so I don't tend to write them as often.