For me interruptions are really, really harmful. I try to focus several hours in a row or almost the full day in a row as much as possible.
I just check the mail in checkpoints, 2-3 max. 4 times per day if I am focused bc anything else would be too productivity-lowering.
I think I am more sensitive to interruptions than most people maybe? This is mainly when programming. For other tasks it is a bit more tolerable.