create email id under my custom domain

Hi,

Could you please provide more information.

1. Detailed version of your exchange server, Get-ExchangeServer | select Admindisplayversion
2. Are you trying to add a new domain? Please refer for adding accepted domains https://video2.skills-academy.com/en-us/exchange/mail-flow/accepted-domains/accepted-domain-procedures?view=exchserver-2019
3. You can create email address policy to stamp the email addresses, please refer https://video2.skills-academy.com/en-us/exchange/email-addresses-and-address-books/email-address-policies/email-address-policies?view=exchserver-2019
4. Different level of limits can be configured, https://video2.skills-academy.com/en-us/exchange/mail-flow/message-size-limits?view=exchserver-2019

In Exchange, only the primary email address is used for emails that you send or reply to.
Follow the steps here and change the primary email address as your custom domain: https://video2.skills-academy.com/en-us/exchange/configure-exchange-to-accept-mail-for-multiple-authoritative-domains-exchange-2013-help?redirectedfrom=MSDN
Besides, if you want to receive messages sent to your new custom domain: An MX record in public DNS is required for each SMTP domain for which you accept email from the Internet. Each MX record should resolve to the Internet-facing server that receives email for your organization.