Is there any update on this?

If nothing is in the filter log, then Thunderbird isn't doing it.

However, test that the filter log is working. Create a trivial filter and with filter logging enabled trigger the filter. Is there an entry in filter log?

If not, then likely your filter log is corrupted. find and delete filterlog.html

Ryan, I overlooked your mention of Apple Mail. Auto reply could be set there. Check settings > apps > mail > accounts > etc.

Do you access this account from other devices or apps?

Hello,

This is still showing no resolution. It does appear the issue is either one.com, however there is no sign of the auto response on there.

Is there another solution to this? Do I contact them?

Hi Ryan,

"I have access to it on my Laptop, again using thunderbird, and my Phone which is Apple Mail. I cannot see anything attached to any of these."

Have you done everything on your laptop's installation of Thunderbird that we have suggested you do on your (apparent) desktop computer?

Did you check Apple Mail on your phone for an auto reply message?

Hello,

Yes, everything has been carried out on both Mac's and nothing is showing on apple mail.

Are you using an add-on to generate an auto-reply?

If not ...

In your place, I would close Thunderbird on my laptop, turn off my phone (not put it to sleep), create a new profile in Thunderbird, set up my account there, open Thunderbird with the new profile, close the instance of Thunderbird with your old profile, and send a message to myself to test the auto reply.

You can manage profiles here: "Help" menu > troubleshooting information > about:profiles

If an auto-reply is generated, it must be on the server.

If an auto-replay is not generated, open Thunderbird on your lap-top computer and repeat the test.

Then close Thunderbird on your lap-top computer, turn on your phone, open Mail, and repeat the test.

This process should isolate the source of the auto reply message.