"回信"是指回复收到的信件或电子邮件。在英语中，可以使用以下表达来表示这个意思：

Reply to a letter:

"I will reply to your letter as soon as possible." (我会尽快回复你的信。)
"I have repped to your letter and sent it back to you." (我已经回复了你的信并将其寄回给你。)

Reply to an email:

"I will reply to your email as soon as I can." (我会尽快回复你的电子邮件。)
"I have repped to your email and sent it back to you." (我已经回复了你的电子邮件并将其寄回给你。)

这些表达来源于英语的常用语言，通常用于书信或电子邮件的回复。