Error saying "Cannot be sent", but mail sent

Today I tried sending an e-mail from my android phone and got a message saying “Failed to send some messages: Unable to send message”. Clicking the error brings me to the outbox with the message I was trying to send in the queue. I could find no information or log about what specifically went wrong that it couldn’t send, like certificate error or what not. I assumed based on the messages, that the e-mail did not send.

I got home and from my PC, I checked my mail and was surprised to have received a response to the e-mail that supposedly hadn’t sent. I checked my Sent Mail folder on my PC and the message still in the Outbox folder of my phone (and error saying failed to send still in my notifications), is found in the Sent Messages box of my PC.

This is IMAP account. Samsung S22 phone with version 12.1 installed. PC is Ubuntu with Thunderbird client, although I don’t think that matters. Not sure what other details I can provide to help understand what might be happening.

Thanks