Unfortunately, I wasn't able to read the e-mail in full, but as I understood, he wanted to apologise for what had happened between us many years ago.

in 在这里是介词，表示"以...方式"或"处于...状态"。 full 是形容词，意思是"完整的、全部的"。 in full 是一个固定搭配的介词短语，意思是"完整地、全部地"，表示以完整的形式或状态进行某事。