How to identify and extract patterns from emails
It depends exactly on which kind of patterns you are talking about. Are they deterministic? That is, they are all the same, so you want to get everything after Dear, or before Att / Best Regards, you can explore regular expression patterns. In python, you can use re library:

https://docs.python.org/3/library/re.html

There are books about regular expressions, so this is a big topic and a strong start if you want to check patterns.

Other kind of tasks you could do:


Word Embeddings on raw e-mails to check word similarity.
Clustering e-mails using Bag-of-Words or Word Embeddings.
Label af set o e-mails and train a supervised algorithm to predict an outcome of interest.


Anyway, text mining is really a big discipline, so if you want a more precise answer, I would recommend to narrow down your question.

Hope it helps.