Please note that in that times, characters were written from right to left.
The postmark says
廿六年 六月 廿一
Which means 21st June, 1937 (literally, 21st, June, 26nd year. Republic of China was founded in 1912. Hence 1912 was the first year and 1937 was the 26nd year.).
The big characters in the top are
安徽 Anhui province
The big characters in the bottom are
揚溪鎮 Yangxi town
Please look at the first character carefully by yourself. It may be 楊 as 楊 and 揚 are very very similar. Searching on the internet, I find there is a town by the name of 揚溪鎮 (simplified Chinese: 扬溪镇) in the county of 绩溪 (Jixi), prefecture-level city of 宣城(Xuancheng) , 安徽 province. Of course, there is still a chance that the first character is 楊 if the original name has been changed.