Ask HN: What does Congress do to verify constituent communications?

Given the openness of communication with any elected official in the USA (see https://www.usa.gov/elected-officials/) what does Congress do, if anything, to verify the legitimacy of calls, letters, emails, etc that they receive from 'constituents'? If these channels are as vulnerable as they seem, does anyone know if any groups, foreign or domestic, have executed a systematic campaign of disinformation directed at Congresspeople?