Humble first-person pronoun used mainly by men in formal letters. Conveys modesty and respect toward the recipient.
I visited your company the other day.
It is due to my own lack of virtue.
小生 is a humble first-person pronoun men use mainly in letters, roughly matching the formality of 不肖 but slightly less self-deprecating.
From Chinese 小生, originally “young or junior person.” It was later adopted as a male first-person expression, commonly used with a self-effacing tone in formal writing.