People applying as common-law have to have two solemnized letters from others attesting that their relationship is real and continuing. They can send more if they want.
People applying as married do not need these solemnized letters, but adding them will not hurt your application. Many married applicant include such letters in their application. The letters can be affidavits, letters, or emails - affidavits are considered the best evidence, but if it is inconvenient or too costly to get all affidavits, letters and emails are fine for married applicants.