Persistent smell on cotton t shrt

One of my boyfriend’s cotton t shirts (possibly with polyester thread) has a particularly bad smell in the armpits that won’t come out. I’ve washed it 4 times, including scrubbing it before putting it in the laundry with bar soap, dish soap and hand soap. I even tried boiling it in a pot on the stove. Nothing has worked. The t shirt still stinks! Is there anything that works for this kind of persistent smell? I can’t soak it in bleach, since it is a dark color, and don’t know if that would work. I have a sensitive nose and this may happen with other shirts, so I’m interested in your work! Thank you!