like most others, I've had this issue. and I've resolved it. Twice, now; in fact; 2 different ways (which is why I'm making this a new post instead of trying to track down all the different posts about this)
I've found (and read about) 2 possible causes to look at to solve it on your own (I'm not issuing exact instructions because different manufacturers have custom builds): My phone was set to poll once an hour and once every half hour (different accounts), and was working correctly, before this upgrade. Problem: the new "Doze" function. Work-around: remove K9 from the "doze" list. I found this out right after I was upgraded to 6.0.0- K9 was re-added to the doze list when I was upgraded to 6.0.1, so you may run into this more than once 2nd possible Problem: Android not reading all of the configuration settings for folder polling after the upgrade, even after reboot likely work-around: Examining and changing a folder's polling settings manually to force K9 to reread/refresh the settings. I did this to one account, and polling started for all accounts I have not rebooted my phone to verify it survives re-powering -- You received this message because you are subscribed to the Google Groups "K-9 Mail" group. To unsubscribe from this group and stop receiving emails from it, send an email to k-9-mail+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.