Do you have any idea why I might be getting bounce back emails when replying to emails I've sent, but not on the first email I sent?

Question

I posted this in the Microsoft Community forum and they told me to post it here.

I've written a VBA script that replies to emails with a given subject line in a given folder. I mostly use it to reply to emails that I've sent, meaning I'd send an email, then reply to that email using the tool. Of 850 emails I sent last week, around 95 got undeliverable emails sent to me, even though I was replying to emails that I had sent before that didn't bounce back. The errors in those bounceback emails vary; some are sender side, others are recipient side. I'm trying to figure out how I can be getting undeliverable messages on email addresses to which I've delivered emails successfully in the past, and how I can avoid bouncebacks in the future. Here are a sample of the errors I've received:

"Status code: 550 5.1.1

This error occurs because the sender sent a message to an email address outside of Office 365, but the address is incorrect or doesn't exist at the destination domain."

This is the one I get that makes no sense to me; I did not get this message when I sent the first email, only the second.

"Status code: 550 5.4.1

This error occurred because a message was sent to an email address hosted by Office 365, but the address doesn't exist in the receiving organization's Office 365 directory."

"Status code: 550 5.0.350

The error reported by the receiving server wasn't specific enough to determine the exact nature of the problem."

"Remote Server returned '550 5.5.0 Requested action not taken: mailbox unavailable (S2017062302).'"

"(reason: 550 5.1.1 <redacted>: Recipient address rejected: User unknown in virtual alias table)"

Do you have any idea why I might be getting some of these errors when replying to emails I've sent, but not on the first email I sent?