。
The mail's delayed because of a strike.
Literal
Mail [subject-が] strike-[cause-で] be-delayed-[ongoing-ている]-[explanatory-んです].
スト is the clipped abbreviation of ストライキ ('strike, labor action') — like many foreign words in Japanese, the full form is shortened for casual use. で marks the cause. 遅れている is the ongoing form of 遅れる ('to be late, delayed'). An everyday complaint framed with the explanatory ~んです.