What are your settings for mail.vodafone.ie? According to their instructions, it should send on port 25 with no connection security and no authentication.

What is your security software, and is it set to scan outgoing mail or the TB profile folder?

Thankyou for taking an interest. My settings are as you suggest, and I have just tried to send myself an email with windows defender turned off for my private network with no success. I have no other security that I am aware of that might have stopped me sending emails. I get my broadband through my phone line and I pay vodafone for the (dubious) privilege. Of interest only, up until this problem occurred I was using mail.iol.ie on port 25 as instructed by eir about 6 or 7 months ago which was the remedy for the same problem then. Apparently they had changed the outgoing mail server without telling me! I don't really understand how eir have any say over the outgoing mail server, but on that occasion changing the setting from mail1.eircom.net (port 587) as per their instructions solved the issue. None of these old outgoing mail server settings work now either, so I'm completely stumped as to how to proceed next with my very limited knowledge!

While I understand your desire to get the account working, it's hard to recommend a mail provider that still offers non-secure servers. Plus, I understand eircom charges a monthly fee for their email. Consider switching to one of the many free or paid-for mail services that provide secure, consistent email.

I have every intention of changing my email provider but I'm not sure that is the problem at this time. I'm thinking that it is Thunderbird which is malfunctioning as I can receive and send emails through the webmail function on eir. It is just that I like being able to store my email history actually on my computer rather than relying on my very poor broadband service.

The ability to send and receive on webmail does not mean TB is malfunctioning, as the two methods use different mail servers and settings. The problem is that there are no documented examples that I can find of an eircom account working with any email program.

That sounds very interesting as I have had this eircom account since 2005 originally working with Outlook Express in Win 98 and then from 2010 on Win Live Mail in Win7 until mid 2019 when I transferred all my archives (with difficulty but with help from the Mozilla community) to Thunderbird in Win10 where it has been working up until a few days ago, when it stopped working with outgoing mail with no clue as to why. I am at my wits end and was considering re-installing Thunderbird until I received your last post which leaves me thinking it isn't Thunderbird at all. I've no idea what to do next! I am intending to try to find a similar email address on gmail as I'm disappointed by the lack of technical support from eir which used to be much better.

I have finally found out from vodafone that this issue is only affecting eircom.net email addresses and they say it is an eircom (eir) problem. Eir claim it is a vodafone problem, so basically I can go to hell, but at least I am no longer looking for a solution and will go to a different email provider with all the attendant bother of changing my contact details on websites that I use. Well done eir you have shed a free user with the minimum of effort. Thank you sfhowes for your help and I hope that anyone else who has this problem can find this answer.