I will be out of town until August 22nd.
我将出差，8月22日返回。（out of town可以看作"离开状态，出差中"的意思。用这个表达时不要忘 记写明自己离开的时段。）
Thank you for contacting Stencils Etcetera. I'm sorry, I'm out of the office until February 5th. Your e-mail will be reviewed as soon as possible after my return. Thank you.

感谢您联系斯坦西斯·埃塞特拉。不过很抱歉，2月5日前我都不在公司。回来之后我将立刻查阅您的来信。谢谢您。

I will be in touch with you as soon as possible when I return.
我回来后会尽快与您联络。（这是表示自己一回来就会给对方回复邮件的句子。）
I will be in touch with you upon my return.
我一回来就联络您。
I will read your e-mail when I get back.
我回来后就看您的邮件。