Cannot send/receive email in Apple Mail for Microsoft 365 account

[macOS 15.4.1, Mail.app 16.0] I use Mail.app to access 4 MS 365 accounts. One stopped being able to send/receive recently (soon after I updated from 15.4 to 15.4.1?). Restarting Mail or the MacBook Pro doesn't help. I still have full functionality with the other 3. Near the upper-right corner of the main Mail.app window is the text "Account Error" with a triangle warning symbol. If I click that text, I get a dialog with the message Online Status \ Connections to host (null) on the default ports failed. I also tried going to System Settings -> Internet Accounts -> *problem account* and then toggling the Mail setting off, then back on. Still no luck. I can read/send email with this MS 365 account on my iPhone with no issues. I realize I can fully blow this account away and then re-sync everything, but this is the account with 3GB of mail, and it would be a hassle to do that, so I'm hoping to find another option to this problem. Thanks for any pointers! EDIT:\ Here is a ping to Microsoft 365 SMTP: >ping -c1 smtp.office365.com PING yyz-efz.ms-acdc.office.com (40.99.227.114): 56 data bytes 64 bytes from 40.99.227.114: icmp_seq=0 ttl=236 time=28.277 ms --- yyz-efz.ms-acdc.office.com ping statistics --- 1 packets transmitted, 1 packets received, 0.0% packet loss round-trip min/avg/max/stddev = 28.277/28.277/28.277/nan ms