Regarding the matter in question, I will explain in detail in a later correspondence.
Literal
Problem [to-と] has-become incident [regarding-については], later-correspondence [by-で] in-detail humbly-say.
Formal business register. 申し上げます is the humble form of 言う, used in keigo to lower oneself before the addressee. 後便 ('later correspondence') is a formal written term. ~については ('regarding') marks the topic of discussion.