When I first visited Westminster, MPS might get a few items of mail a day. Some progressive MPs would hold regular 'surgeries' where constituents could come to meet them to ask for help in solving a particular problem.
I've seen volumes of post sky-rocket - and almost all MPs now do regular surgeries. I was pleased that after my (unsuccessful) runs for Parliament - the sitting MP upped the level of opportunities that his (sadly, not to be mine) constituents had to see him.
...and then there are emails. Free to send, and easier to send than 'snail-mail'. From zero these have become a major part of MPs' workload.
Lobbying groups make a major contribution - and the standard email about a particular campaign are easily recognised. The House of Commons this week issued a Standard Note about a particular email which keeps popping up.