To create a professional email address on your custom domain, such as [email protected], you first need to get a domain name. (eg [email protected])
Secondly, you need to get email hosting. I think the easiest (for those not technically inclined) is to buy from reputable domain hosting companies such as NameCheap, Godaddy, etc, where you can choose the plan that suits you. You would then be able to create an email within the email hosting service.
Another option is to create a professional email address within the web hosting. This is suitable for those who already have a web hosting subscription. I go with this option because I also use self-hosted WordPress. I do not need to pay extra by using my existing web hosting. I have the freedom to set the space (within the hosting plan limit) and create any alias that I like.
Another alternative is to use the forever-free plan from Zoho Mail. However, the free plan has smaller space/attachment limitations. (5GB/User, 25MB attachment limit)