I'm trying to send an email on behalf of my user using nodemailer but all my emails are being rejected.

My transporter works for normal use-cases where I'm sending from my own domain:

const transporter = nodemailer.createTransport({
    host: 'emailhost.com',
    port: 465,
    secure: true, 
    auth: {...},
    dkim: {
      domainName: <domainname>,
      keySelector: <keyselector>,
      privateKey: <privatekey>,
    },
  });

transporter.sendMail({
  subject: "Shop subject",
  from: `kyle@mydomain.com`,
  to: <customerEmail>,
  replyTo: <shopEmail>,
  html: "<div>my user message</div>",
})

But when I try to send on behalf of my user it doesn't work:

transporter.sendMail({
  subject: "Shop subject",
  from: `shop@exampleshop.com`,
  to: <customerEmail>,
  // sender: `kyle@mydomain.com` <-- Also tried adding this field,
  replyTo: `shop@exampleshop.com`,
  html: "<div>my user message</div>",
})

>> "Can't send mail - all recipients were rejected: 553 <shop@exampleshop.com>: Sender address rejected: not owned by user kyle@mydomain.com"

I figured if I set a DKIM DNS record and an SPF record on exampleshop.com it would mean I could send emails on behalf of the domain but still no luck.

Type: TXT Record 
Host: @ 
Value: v=spf1 include:spf.emailhost.com ~all

// Also tried adding this record in place of the one above
Type: TXT Record 
Host: @ 
Value: v=spf1 include:spf.mydomain.com ~all

Type: TXT Record 
Host: default._domainkey
Value: v=DKIM1;k=rsa;p=<DKIM DNS record from namecheap>

It's also been over an hour since I've updated the DNS records, I'm not sure what I'm doing wrong here.