You will probably have to manually modify the character encoding used on those mails when you see them.

View menu (alt+V) > Text encoding

This is commonly seen when mail is sent from old mail clients (outlook express and Eudora being two) and when folk in the USA insist on using the Western character encoding.

Hi Matt, I'm afraid this isn't the answer. I have the same edition on my laptop also with windows 10 and it doesn't have this problem even though all the settings in Thunderbird are the same. I think this Thunderbird on this computer may be corrupted in some slight way. How would you suggest I address that problem?

Stall said

Hi Matt, I'm afraid this isn't the answer. I have the same edition on my laptop also with windows 10 and it doesn't have this problem even though all the settings in thunderbird are the same. I think this thunderbird on this computer may be corrupted in some slight way. How would you suggest I address that problem?

I don't understand what you mean?

This line has no leading space. So this text wraps automatically at the right-hand margin to fit into the display.

This line has a leading space. If you type with a leading space, tab or indent, this forum formats it "as-is". This is useful for verbatim code listings but a nuisance otherwise. 

If you think Thunderbird is corrupt, just re-install it. I doubt that doing this will help. Re-installing does not affect your message store and settings; they are all in your profile.

https://support.mozilla.org/en-US/kb/profiles-tb

Hi Zenos, perhaps you could suggest a solution to my e-mail problem rather than being negative? Can one alter the profile?

I'm sorry, but there may be no simple fix. If someone else's email client is assembling messages improperly then these messages are broken and there's little can be done to remedy things.

Is it always the same sender or group of senders? Do they have an email client in common?

A look at the source of an affected message can be useful in learning what has happened, though it doesn't necessarily lead on to a repair. You may be able to edit the mailbox file and correct what has been done improperly, but TBH I'm out of my depth. :-(

Ctrl+u will show the raw message. Of particular interest are lines that mention encoding or character sets. As an example, these lines are from the notification email sent by the forum to announce your reply:

Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit

Content-Type: text/html; charset="utf-8"
Content-Transfer-Encoding: 7bit

I'd be pleased if a re-install fixed it, but experience shows that this usually doesn't help. If there is an inappropriate setting, that will be stored in the profile, which is the one part of Thunderbird that is retained between updates of the executable program. So settings errors tend to persist. Not that, in this case, I think you have any settings that cause this effect.

I see a lot of websites with this same problem. There is some very broken software out there and some of it may be employed in constructing email messages.

Thanks for your honesty Zenos, I am out of my depth too. It does not seem to be any one e-mail sender which I have the problem with but many, suggesting the problem is at my end. I have set the default incoming mail to Unicode simply because I can get the emails to right themselves by using alt-V and changing the code to Unicode but it does not seem to make any difference. Thanks again for your help, I'll keep trying things.

At this point, you have not mentioned what is in the source code.

Select one of those emails: click on 'More' and select 'View Source'

Scroll down through message and look for any lines that says the same as or similar to: eg: Content-Type: multipart/alternative; MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Content-Type: text/html; charset="utf-8" Content-Transfer-Encoding: 7bit

Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

Post info on what you actually see in those emails.

Hi Toad -Hall, please find what I hope you are looking for, many thanks for your support. _CT, __CTYPE_MULTIPART_ALT,

__CTYPE_HAS_BOUNDARY, __CTYPE_MULTIPART, __BAT_BOUNDARY, __MIME_TEXT_P2,
__MIME_TEXT_H2, __CP_MEDIA_BODY, __HIGHBITS, __FRAUD_CONTACT_ADDY,
ECARD_WORD, __INVOICE_MULTILINGUAL, __INT_PROD_COMP, __SUBJ_ALPHA_NEGATE,
__FORWARDED_MSG, __HAS_HTML, __HTML_TAG_DIV, HTML_NO_HTTP,
BODY_SIZE_10000_PLUS, BODYTEXTH_SIZE_3000_MORE, BODYTEXTH_SIZE_10000_LESS,
__MIME_TEXT_H1, __MIME_TEXT_P1, __MIME_HTML, __TAG_EXISTS_HTML, NO_URI_FOUND,
NO_CTA_URI_FOUND, IN_REP_TO, __MIME_TEXT_H, __MIME_TEXT_P, REFERENCES,
NO_URI_HTTPS, MSG_THREAD, __TO_REAL_NAMES, LEGITIMATE_SIGNS,
LEGITIMATE_NEGATE

That is not the info I asked you to look for. It looks like you have supplied the info that is often in the headers and I've seen under section X-Junkmail-Premium-Raw:

You are definately looking in the right place, but none of that info looks remotely like the examples which both Zenos and myself mentioned. Try again.