Create detail pagecustom button account named as send email

and select execute Javascript and onclick kjava script as 

window.open('/apex/sendEmailFromAccount?accid={!Account.Id}');

vf page :

<apex:page controller="sendEmailFromAccount" showHeader="False" sidebar="false">
 
  <apex:form >
      <center>
        <apex:pageBlock >
        
         <apex:outputLabel >To:</apex:outputLabel> <apex:inputText Value="{!toAddr}"/> <br/>
          <apex:outputLabel >Subject:</apex:outputLabel> <apex:inputTextarea Value="{!toSub}"/><br/>
          <apex:outputLabel >Body:</apex:outputLabel> <apex:inputText Value="{!toBody}"/>
          <apex:pageBlockButtons >
          <apex:commandButton value="send" action="{!sendEmail}"/>
         </apex:pageBlockButtons>
        </apex:pageBlock>
  </center>
  </apex:form>
</apex:page>

Apex controller:

public with sharing class sendEmailFromAccount {


    public String toBody { get; set; }

    public String toSub { get; set; }

    public String toAddr { get; set; }
    
    public  String accId  {get;set;}
    
    public List<Attachment> attList = new List<Attachment>();
    
    public Account accDetails{get;set;}

 public sendEmailFromAccount(){
 accId  = Apexpages.currentpage().getParameters().get('accid');
 accDetails = [Select id, email__c from Account where id=:accId];
 
 }

    public PageReference sendEmail() {
      
      Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
      String[] toAddresses = new string[]{toAddr};
       email.setSubject( toSub );
       email.setToAddresses( toAddresses );
       email.setPlainTextBody( toBody );
       Messaging.SendEmailResult [] srlist =  Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email});   
       for(Messaging.SendEmailResult sr: srlist) {
          if(sr.isSuccess()){
          
            Attachment attOne = new Attachment();
            attOne .body =Blob.valueOf(toBody);
            attOne.parentId = accId;
            attList.add(attOne);
          }
        
       }
       
        try{
            insert attList;
         }
         
        catch(Exception ex){
         
        }
        return null;
    }

}

if you want to send multiple emails, use mass email messages instead of single email message service.