Infinite loop in the on_message method

Hello everyone. I have an infinite loop in my discord bot, but I do not know how to solve it.

import discord
import asyncio
import strings_codes #minha classe de string e códigos
import auxiliary_functions #minha classe de funções auxiliares


client = discord.Client()


@client.event
async def on_ready():
    await client.send_message(client.get_channel(strings_codes.test_channel_id), strings_codes.welcome_message)


@client.event
async def on_message(message):
        lower_command = message.content.lower()

        if lower_command.startswith('!'):
            await client.send_message(message.channel, auxiliary_functions.switch_command(message))
        else:
            await client.send_message(message.channel, strings_codes.invalid_commands['wrong_prefix'])

client.run(strings_codes.token)

The on_ready() method calls the send_message() method with the greeting message and triggers the on_message() method, which also has the send_message() method in its scope, thus making the bot infinitely capturing the your own message. I've seen many people code and they did not have to put a condition to trigger the event that has the on_message() method. How do I solve this? Thankful.