SMTP Library with Custom Domains for Python

Question

I'm trying to log into the SMTP server. The custom domain is not able to be recognized through the smtplib.SMTP host parameter.

Have Tried:

SMTP host for gmail domains

    # login smtp server
    # Issue
    server = smtplib.SMTP(host="smtp.gmail.com", port=587)
    #

    print(server)
    server.ehlo()
    server.starttls()
    print(username, email, app_password)
    server.login(username, app_password)
    print("logged in")
    server.sendmail(from_email, to_emails, msg_str)
    server.quit()

Not Working:

SMTP host for custom domain

    # login smtp server
    # Issue
    server = smtplib.SMTP(host="smtp.customDomain.io", port=587)
    #

    print(server)
    server.ehlo()
    server.starttls()
    print(username, email, app_password)
    server.login(username, app_password)
    print("logged in")
    server.sendmail(from_email, to_emails, msg_str)
    server.quit()