Intermittent problem sending emails

My wife and I both frequently find that emails cannot be sent, with a "server configuration error" message. We both use Thunderbird 52.2.1 (64-bit) on Lubuntu 16.04, so there are no Windows issues. For some time there have been occasional problems giving rise to this error message, but now it is fairly persistent and sometimes we have to wait for several hours before the message can be sent. The accounts are on outlook.com, with server name smtp-mail.outlook.com. The server address is the one found automatically from the database when adding the account. Doing nslookup in a terminal window gives Server: 127.0.1.1 Address: 127.0.1.1#53 Non-authoritative answer: smtp-mail.outlook.com canonical name = smtp.live.com. smtp.live.com canonical name = smtp.glbdns2.microsoft.com. smtp.glbdns2.microsoft.com canonical name = smtp.office365.com. smtp.office365.com canonical name = smtp.outlook.office365.com. smtp.outlook.office365.com canonical name = outlook.office365.com. outlook.office365.com canonical name = outlook.ha.office365.com. outlook.ha.office365.com canonical name = outlook.ms-acdc.office.com. Name: outlook.ms-acdc.office.com Address: 40.101.125.34 ... where the dots indicate a variable number of similar name/address pairs. I've tried resetting the outgoing server, and if I do this then I can send messages, but when I close and then reopen Thunderbird the problem will have returned (again intermittently). I've also tried deleting the account from Thunderbird completely, and then recreating it, and then again I can send messages but when Thunderbird is reopened the problem will have returned. Sometimes I can send messages but my wife can't (both using the same wifi connection). We each have a second account with a different server, and there's no problem sending messages on the second account. Any help would be appreciated.