File no longer available on server

[#1]

This morning my IFA  sent an important message to me, but on trying to open, it was unavailable, as in title.

From: System Administrator <root@localhost>
Subject: message unavailable

This message is no longer available on the server

The above was a text file attachment refused , See box below

Your content was last auto-saved at 1:02 PM



The contents of the attachment does not match its file type

What is cause of this event, and the cure, please? BTW my email client is Thunderbird .

[#2]

Some e mail clients allow RECALL of a message if unopened on server. Get them to resend.

[#3]

I have received replies from Mozillanine  Forum one of which is below

Sometimes that can occur due to an admin renumbering the UID 
(unique identifier for the message) for some of your messages,
 invalidating the UID of the original message. That is outside
 of Thunderbird's control if it occurs before Thunderbird ever has a chance to open the message.

Unfortunately the best way to prevent this is to use a better email provider.
 I'm not a fan of Hotmail but I suspect you're less likely to have problems
 with it than your ISP. ISPs are frequently poor email providers
 because they view email as just a cost center.
 I suggest you have all critical email sent to Hotmail rather than Plusnet in the future.

I have numerous free email accounts (including gmail, outlook, yahoo, comcast, zoho)
 but for almost 20 years have been paying for a inexpensive email account 
with a company that just does email/messaging, which I use for anything important.
 I find it worth the money to get real support and a different attitude towards how they treat users.