Not sure this is an Opalstack issue but... I use Thunderbird for email at home, and when I send email from my Opalstack account (and only my Opalstack account; I also use it for some Google Workspace accounts), I frequently get this message:
"Your message was sent but a copy was not placed in your sent folder (Sent) due to network or file access errors.
You can retry or save the message locally to Local Folders/Sent-[REDACTED]." Options are re-send, save (locally) or cancel.
My internet seems fine and stable in every other way; I'm not sure if this is a hosting issue, or a Thunderbird issue. It happens often enough (several times a week) that it's something I wouldn't mind fixing if it's fixable.
I host a standalone mailserver on Opalstack, if that matters.
