Additional Information which may be of use to anyone reading this question.

I'm with BT and have no issues, but I did have one occasion when access was only available via bt webmail. I was getting this error in Thunderbird: "mail server responded server error - please try later"

Well after a couple of days of this irritating message and no access via Thunderbird.............. Note: I could easily logon to the standard webmail : https://home.bt.com/login/loginform

Then I discovered there were two places I needed to logon to webmail. BT help does not tell you this.

It turned out that the BT and Yahoo servers are not in synch.

So I used this link to logon: https://login.yahoo.com/

Reset the password to the same password I was currently using to access bt webmail and which is also the same as being used in Thunderbird bt mail account.

If for some reason, you are redirected to anything other than what that link says above, then clear your browser cache and re try.

Once I had done this, the yahoo and bt servers were back in sync and everything has been good since. I did not need to change passwords.

I have tried numerous occasions, logging in first to Yahoo mail and then logging in to BT mail. It did not work at all. So I complained to BT in a very aggressive manner, and lo it worked. BT informed me that a " partners server had a fault and now it is rectified. I tested it for some days and it seems they have solved the problem. At last.

I also have this problem, BT denies changing anything and I have been using Thunderbird for years and then I started getting messages in regards an invalid IMAP4 command, going through BT support is basically like swimming through treacle, next to useless, does anyone have a resolution to this issue please. Any help appreciated, desperate & stressed out !!

It would seem you are not alone, several people are reporting this same issue on the bt community website. https://community.bt.com/t5/Email/Unrecognised-IMAP4-command/td-p/1959282

Check you are using server name: mail.btinternet.com for both IMAP & SMTP servers.

Check and reselect: Menu icon >Options >Advanced> 'Network & Disc Space' tab, click connection 'Settings' button, check " Use System Proxy Settings" is selected and then click OK.

Check you have Trash folder as subscribed by doing it again. Right click on mail account name in folder Pane and select 'Subscribe' click on 'Refresh' button. select 'Trash' and click on 'Subscribe' click on 'OK'

I no longer have any email problems as I have changed my email server to Microsoft.... works all the time.... no more lies from BT. Took me 2 weeks to change all my contact details but it was worth it. Also moved to Sky for my broadband and saved £20 a month.

Considering exactly the same can't go yet as my contract runs to April 2020, else I would drop them like a brick, and yes they are liars of that there is no doubt.

There is a possiblity the IMAP4 issue could be related to a CalDav issue, so please try this and report back: Menu icon > Options > Advanced > 'General' tab click on 'Config Editor' button It will say be careful :)

In search type : cookie look for this line: network.cookie.same-site.enabled double click on that line to toggle the 'True' to a 'False' close window - top right x click on 'OK' Restart Thunderbird.

I'm with BT and have the last setting set to true, but I'm not using Imap nor syncing with Contacts etc , so have not experienced the IMAP4 issue.

I am a BT customer and have had this problem several times in the last few years up to the point of changing my email supplier. I have made a lot of noise with BT, both on the phone ( the best way ), and online ( NOT the best way ). It is working at the moment but I do not trust it in the least, but Ihave to have one address for my BT account. After numerous emails, heated telephone calls over period of a month, it ALLWAYS comes back to the same reason ( with apolagys ), that they (BT) had problems with "a third party supplier" not syncing with BT. This usualy means Yahoo who run BT Mail are making a pigs ear out of it AGAIN. I had to threaten to leave after 15 years being a loyal customer even to get them to do something, bad news.

Robert, likewise really, I have been in IT for over 30 years when I announce this the attitude changes, I did get a so called Engineer to state quite clearly that the fault lied firmly in BT's domain, this Engineer was then removed from the process. Unfortunately I find the support staff within BT to be naive and very poorly trained especially the front of office, in all honesty in the 15-20 years I have been with BT and after suffering with many BT caused problems I have only once spoken to someone who I consider competent, it's a shame but BT remain the same and as far as I am concerned have made no progress in regards improvements and if anything the support process has now become pathetic, I had to move from IMAP to POP3 simply from a health point of view, they were simply driving me insane, I remain using POP3 and shall do so for the next 3 months or so until I consider that there is stability with IMAP, regardless of their sad and somewhat pathetic excuses and indeed denials they do not have any lessons learnt, their processes and procedures if they have any do not get updated and pertinent training does not happen. I am awaiting the expiry of my contract with BT then I shall try elsewhere and good riddance to a very inept failing Business.

Thanks for all your replies to my comment about BT email. It became unusable so I changed my email supplier to Microsoft. This has cured the problem completely and outlook.com is totally reliable. I changed 6 months ago now and can use my new email address with ANY mail reader. I wish the best of luck to those with BT email problems. I suffered for 2 years and after being with BT for 50 years it was a nightmare to swap email provider..... but it was certainly worth it. Problem solved.

Shame on BT I say, a company who has lost customer focus, sad and pathetic.