'By my life!' is an exclamation of surprise, something like the modern 'Oh my God!' - and Malvolio is surprised because he has recognised what he believes to be his lady's 'hand' (which means handwriting). Of course, it's actually Maria's handwriting, so (rather like the main Viola-Sebastian plot of Twelfth Night) Malvolio is actually faced with the disguised 'twin' of his lady's hand.
Malvolio is recognising the way Olivia (his lady) writes particular letters: and strengthening his thesis that this letter must have been written by his lady.
However, Shakespeare (or Maria, who has written the letter) has slipped in two very dirty jokes at Malvolio's expense. 'Cut' was the Elizabethan equivalent of the curseword 'cunt' meaning 'vagina' (Donald Sinden in John Barton's RSC production even spelt out her C's, her U's 'N her T's!) and - of course - Olivia 'making her great pees' sounds quite differently read out than it would appear to Malvolio reading it.
These lines (2.5.71-73) can be read as being bawdy. Shakespearian scholar Bruce Smith says that C's and t's are slang for the vagina. (The term was "cut" and Mavolio strings the letters out.) "P," Stewar argues, suggests urine or urination.
Malvolio is Olivia's steward, and it to she he is referring. Malvolio has read forged letter penned by Maria and Sir Toby, designed to make him look foolish. He is convinced that it is Olivia's handwriting, professing her love to someone else. So, on the one hand he might be simply saying he has seen her handwriting frequently enough to know the way Olivia forms her letters. But it is no accident that he chooses *those* letters to focus on. He is incensed and is calling her names.
Ah, isn't this a great scene! I love this silliness.
Okay, this selection has, like many sections in Shakespeare, layered meanings. On the first and most basic level, Malvolio is saying that he recognizes Olivia's handwriting, that's all.
On the next level, this is a source of comedy, because the letter is forged. It is showing that Malvolio doesn't know his "lady" as well as he thinks he does, or himself either. It is scoring on Malvolio, and setting up later jokes. Reading this, we can know that he's going to have something nasty done to him.
On the next level, Shakespeare's being subtly racy. He's slipping in some dirty jokes. Malvolio knows how his mistress makes her P's (her pee)? Yikes! And I will leave it up to your imagination to figure out what dirty jokes can be made with the other letters.