Could you try disabling 2-Step Verification in Gmail, if enabled? See this page for info how to do this, and other steps if it doesn’t work.

Hi,

Thanks. I checked that and it has been turned off.

1. Does it only occur on Windows 10? (see this mozillaZine topic)
2. Does it involve email including attachments, or any email?
3. Are you sure you are using the same From address (Identity)?
4. Assuming it happens for a single message only, what happens if you include the recipient in another message also sent to other domain recipients?
5. Are you using SMTP server settings as described (SSL: 465 or TLS/STARTTLS: 587) and did you try the other ones?
6. Are you using smtp.googlemail.com or smtp.gmail.com, and did/could you try the second?
7. (Probably not the reason but) could you check the MX record for the recipient’s domain? (see 1, 2)

1. I only have Windows 10. 2. No attachments, its any text email. 3. Yes. Checked that - before I answered. 4. Didn't try that one. 5. I'm using SSL but 995. 6. I'm using pop email, smtp.googlemail and pop.gmail for SSL, different port than you list. 7. Little confused as to how to do that (still).

Thanks for your help!

5. The question was about SMTP but you replied with the POP settings. POP is for incoming and so unlikely to be relevant to your sending issue.

Go down to the bottom of the list of accounts in Account Settings and look for "Outgoing Server (SMTP)" at the end. Select that and you'll find your outgoing settings listed to the right.

4. Please do so - just create a message for that recipient only and see what happens and report the exact error message here, if any. It’s also important for us to know if the domain of the recipient is "exceptional", i.e. chances are for instance you sent a message to 4 gmail addresses and 1 non-gmail address earlier. Also make sure the address doesn’t contain non-ASCII characters, likely in the local part of the address. Come to think of it, make sure the address is configured correctly in any of TB’s address books (Personal or Collected addresses), so you could delete and readd it using an incoming message.

6. Please try smtp.gmail.com, port 587, STARTTLS, normal password, only enter your user name (no @gmail.com). If this doesn’t work, do include @gmail.com.

7. Using link 2 in (1, 2) above, you can follow the links that bring you to this place where you can look up MX records. You could enter the recipient’s domain there to check if anything is wrong with it, causing Gmail to block the sending. First however you need to check what 4 does.

Also make sure you are using a valid From address in Thunderbird. You may have set up an identity (=other Sender address) causing trouble. Therefor make sure the SMTP server (that you may just have edited above) is both selected in Account Settings in the bottom right drop-down AND any identities defined and manageable below that - configurable per account so also available for the Gmail account - are configured to use that specific SMTP server. Doublecheck to make sure it is not a previous/older SMTP server configured that may look the same.

Hi,

I had emailed a contact who said they were going to get in touch with the IT people running the group. Amazingly, I can now send.

I am voting they were either blocking gmail or me specifically. They would have had no chance proving anything I was doing anything illegal and immoral.