Microsoft officially does not support HTML markup in SharePoint calculated fields and they have blocked the execution of HTML markup in calculated fields.
More details at: Handling HTML markup in SharePoint calculated fields
Now, solution for your requirement is based on if you are using classic experience or modern experience:
1. Classic experience:
If you are using classic experience, you can make the email address clickable using Client Side Rendering (CSR)
2. Modern experience:
If you are using modern experience, you can make the email address clickable using JSON formatting.
For example:
{
"$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
"elmType": "div",
"children": [
{
"elmType": "span",
"style": {
"padding-right": "8px"
},
"txtContent": "@currentField"
},
{
"elmType": "a",
"style": {
"text-decoration": "none"
},
"attributes": {
"iconName": "Mail",
"class": "sp-field-quickActions",
"href": {
"operator": "+",
"operands": [
"mailto:",
"@currentField",
"?subject=Task status&body=Hey, how is your task coming along?.\r\n---\r\n",
"Click this link for more info. http://contoso.sharepoint.com/sites/ConferencePrep/Tasks/Prep/DispForm.aspx?ID=",
"[$ID]"
]
}
}
}
]
}
Reference: Add an action button to a field (advanced)