[johnea]

I've used thunderbird for decades.

The biggest "modernization" benefit they could achieve would be making it actually fetch mail!

It will only load new mail into folders when the folder is clicked.

I've looked for years for a replacement, but using the integrated calendar has made finding one difficult.

If anyone knows of a GUI email client, that supports integrated calendar, please tell...

[FriedrichN]

This was something that bothered me too. I found that you need to set mail.server.default.check_all_folders_for_new to true in the config editor (can be found under settings).

[beej71]

Mine definitely fetches mail in the background. All my accounts are IMAP, if that's different. I'll be working away and the notifications pop up. Maybe some setting?