太多种了,列举如下：
1）On/Upon receiving my letter,he wrote back to me.
2) He wrote back to me as soon as he received my letter.
3) He wrote back to me immediately/directly/instantly he received my letter.
4) He wrote back to me the minute/moment/second/instant he received my letter.
5) Hardly/Scarcely had he received my letter when he wrote back to me.
6) No sooner had he received my letter than he wrote back to me.
7) He had hardly/scarcely received my letter when he wrote back to me.
8) He had no sooner received my letter than he wrote back to me.