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Sending messages from Thunderbird via Outlook server results in error

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bob.fannon
bob.fannon

bob.fannon

Feb 14, 2026, 13:01 CST

I have used Thunderbird for a number of years with Plusnet (using POP settings) in the UK. However, Plusnet has transferred their support to a company called Greenby who have made a hash of it and offer very poor support. So, I want to use Outlook as the mail server but I am unable to send any messages via Thunderbird to the Outlook server. I can however download messages with no problems. The error message I get is :- Sending of the message failed. Unable to authenticate to Outgoing server (SMTP) smtp.office365.com. Please check the password and verify the 'Authentication method' in 'Account Settings | Outgoing server (SMTP)'

My default Outgoing Sever settings are :- Server Name : smtp.office365.com Port: 587 User Name: bob.fannon@outlook.com Authentication method: OAuth2 Connection Security: STARTTLS

I have tried deleting the Thunderbird account and re-creating it but still got the same result. I also tried to use my old Icloud account ( having first setup my App Specific password) but got the same result.

bob.fannon Feb 14, 2026, 13:01 CST I have used Thunderbird for a number of years with Plusnet (using POP settings) in the UK. However, Plusnet has transferred their support to a company called Greenby who have made a hash of it and offer very poor support. So, I want to use Outlook as the mail server but I am unable to send any messages via Thunderbird to the Outlook server. I can however download messages with no problems. The error message I get is :- Sending of the message failed. Unable to authenticate to Outgoing server (SMTP) smtp.office365.com. Please check the password and verify the 'Authentication method' in 'Account Settings | Outgoing server (SMTP)' My default Outgoing Sever settings are :- Server Name : smtp.office365.com Port: 587 User Name: bob.fannon@outlook.com Authentication method: OAuth2 Connection Security: STARTTLS I have tried deleting the Thunderbird account and re-creating it but still got the same result. I also tried to use my old Icloud account ( having first setup my App Specific password) but got the same result.

All Replies (2)

david
david
  • Top 10 Contributor

My configuration matches yours, so I have no suggestions, unless the Plusnet comment relates. If so, how? Thank you.

bob.fannon
bob.fannon Umnikazi wombuzo

Thanks for checking that. I wondered whether there was a password element to the problem. From what I can see there is only a password entry under the Outgoing Server setting (and its only a problem when sending messages). However, surely there should be a password check when downloading messages ? Having said all that, I have used the same password for accessing my Outlook webmail without any problems. I am sure there is no Plusnet influence as I have created a new different account on my PC while I try to get it working and not interfere with my existing shaky (Greenby/Plusnet) arrangement. Is there a way to involve Mozilla support do you know ? Thanks in advance, Bob

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