Yeah Zelle doesn't really work well for that because I think you can only ever have one Zelle-enabled account because it's based on phone number or email.

Some banks offer external bank transfers through their online portal, and sometimes with a small transaction fee. I've been told by my bank with that transaction fee that you can avoid it by using the "bill pay" feature and just inputting your other bank account's routing and account information. I think that all works over ACH anyways.

I was hoping to use my catch all domain to resolve that e.g. [email protected], [email protected] which is what I do with Interac. Looks like it got me flagged.

Most bank accounts I've ever had in the US let you set up bank-to-bank transfers via the web site. It only requires a routing number and an account number.

TD too but that takes 24-48 hours and you have to go through a weird validation process. Two deposits are made to the receiving bank under and you're asked to confirm.

TD also wanted me to come into a branch with a bank statement from my other bank. Not ideal since they don't operate west of South Carolina and I'm in California.

The two deposit thing is pretty standard. A little odd, but it gets the job done and presumably does so using existing ACH interfaces (so they don't need to implement anything new or rely on the receiving bank to do the same)

Most services I use integrate Plaid. I believe Plaid is just a federated authentication glue that accesses your bank's systems and is granted access to account balances and ACH details (routing/bank account numbers).

It always struck me as an odd product, since I can just plug my ACH info in directly, but it does provide some level of convenience by allowing, say, a roboadviser app to show the embedded balances of my other accounts.

Plaid is a disaster waiting to happen—they store your banking username and password, and then use them to log in to your online banking on your behalf where they scrape the info they need.

I’m amazed banks put up with it.

The Plaid disaster hasn't waited- it's already started. They've already had to settle at least one class action lawsuit for deceptive practices

https://www.plaidsettlement.com/

It'll use app-specific passwords or OAuth with banks that support that, and both of those can be secured well enough.