What are the settings as per your provider's information?

Please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information. Press the Copy text to clipboard button and paste the information into your reply.

Thanks for your reply. Copying the troubleshooting info below. I've updated account no. 3 to the new details from my ISP. Others are all still set at old details from the various email providers for my different accounts. All receiving fine, not able to send from any for the last 2 days. Before that working fine.

 Application Basics

   Name: Thunderbird
   Version: 38.7.0
   User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:38.0) Gecko/20100101 Thunderbird/38.7.0
   Profile Folder: Show in Finder
   Application Build ID: 20160310150025
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (imap) imap.smtp.cz:143, alwaysSTARTTLS, passwordCleartext
     OUTGOING: out.smtp.cz:587, alwaysSTARTTLS, passwordCleartext, true
     OUTGOING: out.smtp.cz:587, alwaysSTARTTLS, passwordCleartext, false
     OUTGOING: out.smtp.cz:587, alwaysSTARTTLS, passwordCleartext, false

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account3:
     INCOMING: account3, , (imap) imap.smtp.cz:143, alwaysSTARTTLS, passwordCleartext
     OUTGOING: smtp.airwaynet.cz:22, plain, none, true

   account4:
     INCOMING: account4, , (imap) imap.pyt.cz:143, alwaysSTARTTLS, passwordCleartext
     OUTGOING: smtp.pyt.cz:587, alwaysSTARTTLS, passwordEncrypted, true

   account5:
     INCOMING: account5, , (imap) imap.smtp.cz:143, alwaysSTARTTLS, passwordCleartext
     OUTGOING: out.smtp.cz:587, alwaysSTARTTLS, passwordCleartext, true

   account6:
     INCOMING: account6, , (imap) imap.smtp.cz:143, alwaysSTARTTLS, passwordCleartext
     OUTGOING: out.smtp.cz:587, alwaysSTARTTLS, passwordCleartext, true

   account7:
     INCOMING: account7, , (imap) imap.smtp.cz:143, alwaysSTARTTLS, passwordCleartext
     OUTGOING: out.smtp.cz:587, alwaysSTARTTLS, passwordCleartext, true

   account8:
     INCOMING: account8, , (imap) imap.smtp.cz:143, alwaysSTARTTLS, passwordCleartext
     OUTGOING: out.smtp.cz:587, alwaysSTARTTLS, passwordCleartext, true

 Crash Reports

 Extensions
   Account Colors, 9.2, true, accountcolors@DW-dev
   Manually sort folders, 1.1, true, tbsortfolders@xulforum.org
   Super Date Format, 1.1, true, superdateformat@buildontechnologies.com

 Important Modified Preferences

   Name: Value

     browser.cache.disk.capacity: 358400
     browser.cache.disk.smart_size_cached_value: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     extensions.lastAppVersion: 38.7.0
     mail.openMessageBehavior.version: 1
     mail.spotlight.firstRunDone: true
     mailnews.database.global.datastore.id: 85ae694b-295f-b34d-80c1-200a84fddf5
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1458415546
     places.history.expiration.transient_current_max_pages: 100664
     plugin.importedState: true

 Graphics

     Vendor ID: 0x10de
     Device ID: 0x 8a0
     WebGL Renderer: false
     GPU Accelerated Windows: 0

     AzureCanvasBackend: quartz
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: none
     AzureContentBackend: quartz

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.10.10
     4.10.10

     NSS
     3.19.2.3 Basic ECC
     3.19.2.3 Basic ECC

     NSS Util
     3.19.2.3
     3.19.2.3

     NSS SSL
     3.19.2.3 Basic ECC
     3.19.2.3 Basic ECC

     NSS S/MIME
     3.19.2.3 Basic ECC
     3.19.2.3 Basic ECC

I've updated account no. 3 to the new details from my ISP.

So what are 'the new details from your ISP'?

account3: OUTGOING: smtp.airwaynet.cz:22, plain, none, true

I'd really be surprised if this is what you got from your ISP.

First I reported the issue with the server timing out, they told me they don't block any ports so all should be fine, but to try changing to their server:

smtp.airwaynet.cz and use port 22 with no password, authentication etc.

they actually wrote smtp.airwaynet.cz. (with the . ) so I tried that first, but didn't work either.

They've now said they don't know what to do. That I should try sending via a different network to work out if the problem is local/computer-related, or network-related which I can do tomorrow.

Port 22 is used for SSH. If they seriously told you to use port 22, and no authentication/no password, find a new email provider.

For SMTP there are a number of options:

port 25 normal password, clear text If this is all they offer, find a new email provider.

port 465 normal password, SSL/TLS

port 587 normal password, STARTTLS

In any case, your email provider should be able to tell you the correct settings. It's their server after all. May be airwaynet.cz are an exception.

Thanks for this, I just tried all your suggestions and managed to send email with port 25, encrypted password.

What I still don't understand is why this suddenly became an issue? If you have any insight it would be great!

You'd need to ask that to your email provider. It seems they made changes to their servers without explaining them properly. As said before, I'd look for a new email provider.

In any case, when your problem is fixed, can you mark the thread as 'Solved' please? Thank you.