My guess is you have two SMTP servers configured and different ones are being used for new mail and replies.

Check the from lines in the wrote window and see if there is anything to see there.

Thanks for your reply, Matt.

I did some additional experimentation with other recipient email accounts (non-Hotmail). New messages composed in Thunderbird with the problem account were received without issue.

This got me thinking that the Hotmail account was blocking the emails. I removed my HTML signature and this appears to solve the problem (well, get emails delivered at least).

After some further trial and error, it would appear to be the use of URLs in my signature that causes the emails to be rejected by Hotmail. Oddly enough, they don't cause an issue when sent using the online Gmail interface.

I've taken the path of least resistance and decided to stick with Gmail. It's a shame, so I'm open to trying any ideas people might have.

I recently helped someone with a similar problem regarding url in email.

But it would be useful to know a bit more about the methods you are using and how that signature was created and how it is added to email.

Do you create/compose emails in Plain Text or HTML ? I'm presuming you are problably using HTML.

Check sending options:

• Menu app icon > Preferences > Composition
• click on 'Send Options' button

Suggest you make sure you are sending using both.

• uncheck 'send messages as plain text if possible'
• Select 'Send the message in both plain text and html'
• click on OK

Is the url just text or a clickable image ? Do you allow signature delimiter -- to be included?

How are you adding the signature ? Manually ? In the Account Settings 'Signature text' box? OR Do you use a html file to add signature ? If yes, you do use a html file - Did you use Thunderbird 'Write' window to create the signature, save it as html file in a suitable folder on desktop and then select the checkbox in Account Settings to use that html file ? If no - then I suggest you open a new Write message and completely clear it. Press Enter a couple of times to get a clear space (it looks a lot better and offers an easy clear space when composing a new message) and then type/create your signature. If required use a table to help control positioning of parts of signature. Save as File eg: Formal-sig.html Save in a suitable location. I have a 'Signature' folder which I keep in 'My Documents' and that 'Signature' folder contains all my html signature files and any images I use in signatures. In Account Settings select the option to add signature as html file and choose the file you saved.