email-format | Email data structure and builder for streaming emails | Email library

If you wanted to avoid using regex (which is, in my opinion, difficult to decipher), you could use the .Split() method on the email string using the "@" symbol as your delimiter. Then, you can check the string lengths of the two components from there.

I see some problems there. I think the main problem is, that you try to send Windows-1252 encoded data, but your E-Mail header says, that you have 8 Bit ISO-8859-1 Encoding in mail body. As you might find in Wikipedia, ISO-8859-1 and Windows-1252 are not the same. The most characters are the same, but Windows-1252 is the advanced ISO-8859-1 character set. This might destroy your entire E-Mail. So, please decide, which one you would like to use. I would advice you to prefer ISO-8859-1 over Windows-1252 Encoding, because that's typically for old 8-Bit E-Mails.

Also, the most of the data seems to come from an HTML form. If so, you could simply set the accept-charset attribute in your form Element to ISO-8859-1. So, in other words: Let the client do the hard work for you. But please validate it also on your PHP backend, that the client sends you really ISO-8859-1 encoded form data and something else. Because you cannot trust any client.

Another reason could be, that you first convert from an unknown encoding to UTF-8 and from UTF-8 back to Windows-1252, which could cause also some problems. iconv can handle this in one step for you (I will take the first found line form the top of your code sample as an example):