If you are working with a corporate mail account you may need to confirm settings with the corporate IT support. My own company does not have SMTP turned on for external client software, so I use a 3rd party addon to connect between Thunderbird and their server. In your OWA webmail account check Settings > Mail > POP and IMAP. Confirm settings are on and match your Thunderbird settings. Check your computer firewall and network security allows SMTP traffic - in a corporate system that may be turned off/port closed. It's complicated, I am not expert and just scratching the surface, hoping something will help you. Do get your local IT to help.

Thank you for sharing your experience. What I have checked so far is telnet to that SMTP to xpected port, at least there is responce, so it does not look like firewall issue. Moreover, windows native Mail client does work well. Of course, I have doublechecked server, port, security protocol to those listed in web-based outlook. So it looks something is very specific in a way TB communicates to the server. Company IT admin is a right person to ask, but the point is that I am among those two dinosaurs, who use TB daily to handle my mail. Other people just happy with microsoft bundled solutions. Having that, I wish I trace down the issue, so I could instruct admin where to look. Meanwhile, could you please share a little about that plugin you use to send your mail? Thanks

The plugins are intended to communicate two different ways, when the corporate system does not allow direct SMTP and IMAP/POP. 1) MS Exchange server - use 'Exquilla for Exchange' (I use this one) 2) Through OWA webmail - use 'OWL for Exchange' Both listed in TB Add-ons, both managed and supported by Beonex, both have a small paid subscription. I think for a new setup, Owl may be the way to go if you can't communicate direct. Agnes

Agnes, Thanks for suggestion. I have tried OWL. Can't say I am completely happy, but that is better than nothing. With that plugin I can send meil, but all the ifrastructure of the folders is absent. Interestingly, this plugin is offered on mail account setup screen. I am just curious, is ut in a way possible to have TB operational with office 365 mail, or just give up and search no more?

Honestly, I can't tell you. I work in a system that is using MS Exchange but not O365. I am the minority person (let's say enlightened rather than dinosaur) who wants to use TB, and does not get much support from IT to do so. I can't tell you more about how other systems work, or how the server and and client computer need to be set up, but I do believe it can be done. There are MS articles about this but written for network admin and server managers not the end user. For the OWL setup check it is IMAP not POP, that could explain why you don't see the folder structure. If IMAP, you may have to 'subscribe' to other folders, but I am not sure when you use Owl whether that is necessary. Owl support would be a better source for those questions.