well, if that case, you need to use regular smtp server.... i give you example
if your main domain name is rcchost.com, and you used smtp from sendgrid, you can create subdomain for your main domain and use smtp from your current hosting.
let' say we create whmchs.rcchost.com, you can setup email address "
anyemailaddress@whmcs.rcchost.com", then you setup smtp in your whmcs using
anyemailaddress@whmcs.rcchost.com credential and using smtp server from your current hosting, not from sendgrid.
hope you can get what i mean lol