A user on DLSR posted about an issue he was having with sending Outlook Webmail via Firefox but okay using Chrome. I thought I'd check my end to see if I can duplicate. Sure enough....I'm seeing the same issue.

I can log in to Outlook webmail(the free MS email outlook) and send a test message to one of my Yahoo accounts. The message is logged in the Sent folder but never arrives. I tested using MS Edge. Same thing. I installed Chrome, logged into Outlook.com and message arrives almost immediately.
This is on a Win10 machine. I tried sending a test message using the Win10 version of Outlook(Windows Mail) and message is successfully sent and received. I am an Office 365 subscriber. If I send a test message using the Office 365 Outlook it works fine too.

Can anyone check using Firefox and/or MS Edge to see if this duplicates. Not sure if this is some weird Browser issue or something else.