code:

import dash
import dash_bootstrap_components as dbc
import dash_html_components as html
from dash.dependencies import Input, Output
import requests

app = dash.Dash(external_stylesheets=[dbc.themes.COSMO])

email_input = html.Div(
    [
        dbc.FormGroup(
            [
                dbc.Label("Email"),
                dbc.Input(id="email-input", type="email", value=""),
                dbc.FormText("We only accept gmail..."),
                dbc.FormFeedback(
                    "That looks like a gmail address :-)", valid=True
                ),
                dbc.FormFeedback(
                    "Sorry, we only accept gmail for some reason...",
                    valid=False,
                ),
            ]
        )
    ]
)

app.layout = [dbc.Container(
    dbc.Alert("Hello Bootstrap!", color="success"),
    className="p-5",
),
html.Div(email_input)]

# --- Callbacks --- #
@app.callback(
    [Output("email-input", "valid"), Output("email-input", "invalid")],
    [Input("email-input", "value")],
)
def check_validity(text):
    if text:
        is_gmail = text.endswith("@gmail.com")
        # YOUR API REQUEST
        url="http://example.com/api"
        headers = {"header1":"header_value"}
        response = requests.post(url, headers)
        return is_gmail, not is_gmail
    return False, False

if __name__ == "__main__":
    app.run_server()