AcroSuite Mailer provides Verify Email Address and SendMail APIs.
You may send email by RESTful API with Digital Signature or not.
About Digital Signature, you may make use of AcroSuite Cipher to generate Digital Signature and check its validity.
please refer to AcroSuite Cipher.
https://rapidapi.com/AcroChannel/api/acrosuite-cipher/

##1> Verify Email Address
Verify Syntax and Domain’s MX records of mail address up to 20.
https://acrosuite-mailer.p.rapidapi.com/v0_1/CstService/tools/mail/isvalidaddress
http method: get

Request Header parameters:

x-rapidapi-host: acrosuite-mailer.p.rapidapi.com
x-rapidapi-key: [your rapidapi-key]

Request Query parameters:

emails=[EmailAddress];[EmailAddress]…[EmailAddress]

emails: MailAddress set up to 20. ex: emails=user1@hotmail.com;user2@gmail.com

Response JSON Example:

-Successful case:

{
    "contents": {
        "code": "0000",
        "message": "",
        "subject": "checkEmailAddress"
    },
    "errors": null,
    "memo": "",
    "result": true
}

-Failed case:

{
    "contents": null,
    "errors": [
        {
            "code": "302",
            "message": "A validate format of Email address [e.g.: staff@sample.com] is required! [Invalid HostName] user@hotmaail.com",
            "type": "ParameterError"
        }
    ],
    "memo": "Unfortunately, you failed!",
    "result": false
}

##2> Send Mail
https://acrosuite-mailer.p.rapidapi.com/v0_1/CstService/tools/mail
To define a content type and send Email with Digital Signature or not. Syntax and Domain’s MX records of fromAddress will be verified to prevent spam.

http method: post

Request Header parameters:

Content-Type: application/json
x-rapidapi-host: acrosuite-mailer.p.rapidapi.com
x-rapidapi-key: [your rapidapi-key]

Request JSON Example:

{
    "mailSubject": "Meeting Notice",
    "mailBody": "It is a sample",
    "contentType": "text/plain;charset=UTF-8",
    "toAddress": "********@hotmail.co.jp",
    "ccAddress": "",
    "bccAddress": "",
    "fromAddress": "********@outlook.com",
    "asynFlag": false,
    "userSignature": {
        "publicKey": "H4sIAAA...AA",
        "signature": "H4sIAAA...AA",
        "signDT": "2020-04-27 17:12:31.519",
        "signHash": "lmTcHazlUGh7IY8qS5J1VQ",
        "signName": "User1"
    }
}

   "mailSubject","mailBody": Total size limit is 20MB.
   "contentType": If only a text message will be sent, please set to "text/plain;charset=UTF-8". A html email will require "text/html" contentType.
   "toAddress","ccAddress","bccAddress": Total number limit is 20.
   "fromAddress": Syntax and Domain's MX records of fromAddress will be verified to prevent spam.
                Please tell me your Email Address or Domain for Sender Authentication (fromAddress). 
                If you select Domain Authentication, we will reply you some DNS records. 
                It is necessary that you install those records in your DNS configure.
   "asynFlag": true: send email by async mode, false: send email by sync mode.
   "userSignature": userSignature may be neglected, please view AcroSuite Cipher about its format.
                   https://rapidapi.com/AcroChannel/api/acrosuite-cipher/

Response JSON Example:

-Successful case:

{
    "contents": {
        "code": "0000",
        "message": "The mail has been successfully sent!",
        "subject": "sendMail"
    },
    "errors": null,
    "memo": "",
    "result": true
}

-Failed case:

{
    "contents": null,
    "errors": [
        {
            "code": "760",
            "message": "ToAddress is invalid! [Invalid HostName] xxxxx@hotmailss.co.jp",
            "type": "ParameterError"
        }
    ],
    "memo": "Unfortunately, you failed!",
    "result": false
}