Hello @Patricia1, welcome to the Let's Encrypt community.
That is up to you.
For a Domain Validated certificate YES, however there are other type of certificates for different uses.
Technically you do not create the Certificate, you request the certificate and the Certificate Authority validates that the Certificate can be issued and then issues the Certificate. In Let's Encrypt case they are Domain Validation (DV) certificates so that is what must be validated.
You have not supplied much information, so regarding "Using secure messages from Mail on a Mac"
So what type of secure messing are you using for Mail on a Mac?
Let’s Encrypt offers Domain Validation (DV) certificates.
Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. crt.sh | example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
Thus you need to own and have control over the Domain Name (or have a subdomain under an existing domain name, for example pointed to your server by your employer or school) you wish to obtain a certificate for, from an ICANN Accredited Registrar.
For Let’s Encrypt to issue a Domain Validation (DV) certificate Domain Validation must happen
and it is a CA/Browser Forum Baseline Requirement.
Best Practice - Keep Port 80 Open
What IP addresses does Let’s Encrypt use to validate my web server?
Let’s Encrypt does not publish a list of IP addresses we use to validate,
and these IP addresses may change at any time.
Let's Encrypt uses Multi-Perspective Validation Improves Domain Validation Security - Let's Encrypt
Since these are Domain Validation (DV) certificates the Domain Name System (DNS) is used extensively in the validation process as well a allowing us to assist here on Let's Encrypt community.
DNS Queries need to give consistent results from any location on the Internet, all your authoritative DNS Servers for the Domain need to also give consistent results as well.
Testing and debugging are best done using the Staging Environment as the Rate Limits are much higher. Rate Limits are per week (rolling).
And to assist with debugging there is a great place to start is Let's Debug.