My T will let me know his email availability when he's going on vacation, like "I'm checking and responding once a day" or something like that. He says it's fine to email. On his most recent weeklong vacation, I tried to hold out (having a session with a backup T--was a rough week), but ended up emailing him at one point, but just basically asked if he still existed (he replied the next morning, I think?). When he got back, he said it would have been fine to have contacted him more (seemed kind of sad I was struggling but didn't) and that it was like I was asking for the absolute bare minimum in that email. The way I see it is, it's up to them to decide if they want to offer it. They could always choose to say they're not allowing emails (or only in a crisis, something like that). It's up to them to set their own boundaries. If they offer it, it's OK to use it. I'd talk to your T about this when he gets back though, because it's clearly bothering you.