Two reasons I can immediately think of for causing this to occur. If a contact does not have a valid email address then it cannot be included in the Mailing List. Check the contact in the 'Personal Address Book' to make sure there is a valid email address in the 'Edit Contact' winsow, 'Contact' tab 'Email' field.

If the contact you are trying to add shares the same email address as another contact who is already in the Mailing List then you cannot add the contact in the usual manner; you will need to use a workaround as mentioned here: https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird#w_how-to-add-two-contacts-who-use-the-same-email-address

Nor being able to add or delete from a mailing list sounds like something is corrupted.

TRy the following: export the Personal Address Book to LDIF and store on eg: desktop In Address Book

• select 'Personal Address Book'
• 'Tools' > 'Export'
• A window opens so you can choose where to save eg: Desktop
• 'Filename': Change File name to eg: 'OriginalPAB'
• Save as Type 'LDIF'
• click on 'Save'

In Address Book, delete the Mailing List that was under 'Personal Address Book' Now import the address book back.

• 'Tools > 'Import'
• select 'Address Books'
• click on 'Next'
• select: 'Text file (LDIF, .tab, .csv, .txt)'
• click on 'Next'
• locate the 'OriginalPAB' LDIF file on desktop
• click on 'Open'

This should import back a copy of what you have in Personal Address Book and also the Mailing List. It will just be called 'OriginalPAB'. Can you now add / delete from that imported Mailing List ?

The first reply does not apply.

The second reply. I did as suggested but I am unable to add from OriginalPAB to the mailing list neither can I delete from the mailing list.

This is now resolved.

I decided to delete the problem mailing list and start again by typing in each email address. This was not too onerous because, having typed a few characters, I was able to click on the applicable address. Having entered the whole list (about 35 addresses) I found I could then use the drag method. I can also delete.

I don't know how this happened but it all works correctly now.

By the way, as you say, you cannot add a contact who does not have a valid email address. I wanted all contacts in the mailing list so I got over this by typing A@B.com for a contact without email. Also, as you say, you cannot add a contact with the same email address as one already in the list so the next non-email contact was entered as C@D.com etc.

As a heads up: A 'Mailing List' is designed to be used for sending one email to all contacts in a 'Mailing List', hence the need for a vaild email address. It serves no other purpose.

'Mailing Lists' are not address books. All people in a 'Mailing List' are also in the associated 'Address Book' eg: Personal Address Book. They are not moved into a Mailing List.

All contacts are also visible in 'All Address Books' which is a virtual address book.

When you search for email address/contact, this is done by searching the address books not the Mailing Lists.

When you use the 'Contacts Sidebar' in a Write mesage to locate contacts and auto insert into eg: TO fields they are located in the address book.

You can create additional address books for different groups of people eg: Business, Family, Friends etc.

re : I wanted all contacts in the mailing list. Could I ask you what is the purpose of this as you are not using the Mailing List as a Mailing List and it is not searchable nor able to be used to select individual contacts in a Write window Contacts sidebar? The 'Mailing List' is also rendered useless as a Mailing List as it is full of bogus email addresses. Curiosity has got the better of me :)

Thanks for your comments. I understand all the points you make but, as an octogenarian with no computer training, I have to muddle through and very often my methods are not the most efficient.

I will now create address books for all my groups and use those to make mailing lists.