additional_machers.py

import requests
import requests_mock

def match_limited(request):
    return '"Limit":10' in request.text

def match_limitless(request):
    return '"Limit":10' not in request.text

limited_response = "limited"
limitless_response = "limitless"

with requests_mock.Mocker() as mocker:
    mocker.post("/?Key=MessageGetList",
                additional_matcher=match_limited,
                text=limited_response)

    mocker.post("/?Key=MessageGetList",
                additional_matcher=match_limitless,
                text=limitless_response)

    print(requests.post("http://test.com/?Key=MessageGetList",
                        json={'hello': 'world'}).text)

dynamic_response.py

import requests
import requests_mock

def my_response(request, context):
    if '"Limit":10' in (request.text or ''):
        return 'limited'
    else:
        return "limitless"

with requests_mock.Mocker() as mocker:
    mocker.post("/?Key=MessageGetList", text=my_response)

    print(requests.post("http://test.com/?Key=MessageGetList",
                        json={'hello': 'world'}).text)