Q. Why do the words in my email have red squiggly lines under them?

The red squiggly lines indicate that the word is misspelled.

To correct it if necessary right-click on the word and choose from the suggested spellings in the list, this will remove the squiggly line.

If the word is not misspelled such as would be the case in a persons name then you could right-click and choose "Add to Dictionary".