Grenby migration - able to receive but not send emails

[#1]

I have 2 mailboxes: mb1 - password not changed since migration on 19 Dec - uses Postbox
                                mb2 - password changed after migration - uses Thunderbird
mb1 can send and receive emails
mb2 can receive emails but not send them => authentication failure at relay.plus.net. This has been the case since Dec 19 despite having raised 2 tickets with Geenby "support" ( a misnomer as support from them is as likely as spotting a unicorn in my garden).
I used the curl cmds as generated by a message in this forum. imap succeeded but smtp failed. The encrypted password on both was ****

Today I logged in to my Plusnet account and saw a link to Webmail which I thought was not handled by Greenby. So I went to webmail logged in and was surpised to see that my inbox was up-to-date and I could send an email to myself. On examining the header it was received from mail.enmail.co presumably the Greenby SMTP server. 
Can anyone give me a clue as to what is going on and how can I fix it (my hunch is that since there are now two places where passwords are stored and Greenby is the master, a change in pw on Greenby send a password update to Plusnet where the imap password is changed but not smtp one. 

[#2]

@InSale 

Can anyone give me a clue as to what is going on and how can I fix it (my hunch is that since there are now two places where passwords are stored and Greenby is the master, a change in pw on Greenby send a password update to Plusnet where the imap password is changed but not smtp one. 

Close!

Normally migtated accounts would automatically route incomimg and outgoing connections to the Greenby servers. However, migrated plus.com accounts temporarily still have the outgoing (SMTP) connection routed to the Plusnet server. This will not be aware of any password change made on the Greenby portal.

Hence, try using your existing credentials. If you can't remember any old mailbox passwords then you can use the main account details and password i.e Plusnet account name & password used to access the member centre

[#3]

is set up the same for going direct to Greenby SMTP server as Plusnet webmail does?

[#4]

webmail is hosted on the Greenby servers and will go directly to the Greenby smtp server. You CAN connect directly to the Greenby server and use the changed credentials BUT your email will not currently be DKIM signed

[#5]

What's the implications for me I use Thunderbird as my email client?

[#6]

YBasically you need to use your existing credentials forthe SMTP server  relay.plus.net

[#7]

To your knowledge is Plusnet going to fix the relay.net problem? If they are not, can you suggest a way to configure a Thunderbird email to reliably send emails bearing in mind I have no idea what relay.plus.net is using for each of my mailboxes. In a previous reply you suggested using my Plusnet account login which has yet another password>
i've had some inconsistencies using Greenby SMTP server. mb1 sent message to mb2, message appears in both Plusnet and Greenby Webmail inbox but not in Thunderbird inbox

[#8]

@InSale 

To your knowledge is Plusnet going to fix the relay.net problem?

Yes

If they are not, can you suggest a way to configure a Thunderbird email to reliably send emails bearing in mind I have no idea what relay.plus.net is using for each of my mailboxes.

relay.plus.net does not require that the smtp authentication credentials are the same as the sending email address  

In a previous reply you suggested using my Plusnet account login which has yet another password

Im assuming you must still know that password (even if you dont recall any mailbox passwords) since you still have a Plusnet broadband account, so given the above, you can use the broadband account credentials to authenticate with relay.plus.net