There are some deceiving registrars called 'domain slammers' who send misleading emails urging you to renew your domain with them. Please check Wikipedia for more information about domain slamming.
Such emails are generally considered as 'scam' and you can safely disregard them.
If you are uncertain which is your registrar, you can easily check it by going to whois.sc. When a domain is registered you will see:
Registration Service Provider:
STD Group Inc.
Reported domain slammers:
- Liberty Names of America
- Domain Registry of America
- Domain Registry of Canada