I have to disable it to reply to an email sent to me, however I can send an email that I write to someone.
TIA
That’d be super annoying.
A bit more info please: you can send an email to the same someone that you can not reply too? As in the same email address?
and
What sort of VPN are you using? Corporate or one of the “privacy enhancing” over sold ones.
and
What email client are you using?
Thanks for the reply.
Yes. If I want to initiate and then send an email to somebody, it sends just fine. If somebody sends me an email, I receive the email, but if I go to send a reply I get a pop-op that says ( in very many words ) that it can’t be sent. Temporarily disable VPN, then I can send the reply.
Using Mozilla VPN. E-mail client is Thunderbird.
Weird.
Maybe the VPN is blocking a TCP port. Without more info I cannot help more but if it is anything I suspect it is that. Yet, probably not since if that were the case no email would be going out.
It is odd you can send but not reply. Not sure what the deal is with that. Your email server should view a reply the same as any other email and pass it along. If the receiving server does not like it for some reason that’s their thing, not yours. Hopefully that server would reply with an error message of why it was not delivered but they do not always do that.
I have a VPN and notice that plenty of websites realize I am on a VPN. It is possible the receiving server has a list of VPN IP addresses and just blocks them. But, again, why only on a reply? Can you send to the same email recipient if it is not a reply?
Yes. Hence my puzzlement. 
Does your email provider have a webmail portal? If so, do you see the same issue if you try to reply to an email using that?
Is there anyone else copied on the email? Or just the one recipient?
Also, are you SURE the reply to email can be sent to? Many automated emails will say you cannot reply to them. I mean, you can but no one will ever see it and it will be rejected. Is it something like that?
No, it’s not one of those automated “do not reply” emails, they are personal emails between me and a family member’s PC.
So it’s all replies, or just this one?
Which email provider? (If you don’t mind saying)
Are you trying to send the reply with original as text or an attachment?
I assume you mean it pops up immediately saying that it can’t send? Or is there a delay?
If you copy and paste the entire email into the body of your new composed email does that also fail?
For a while there were issues with some lesser email providers (and even GMail) not being accepted by some servers. However, I thought that was long past. Plus, it makes no sense that only replies are excluded. that suggests it’s the content.
(Read the error carefully - to determine if your server is rejecting it, or if the email server it then sends it to is rejecting it… Usually there’s an eeror code like “521 5.7.1” that gives a clue too.)
I also find that copy-paste the error message into google may help chase down the problem. Google is your all-purpose computer manual.
Is there something in the email reply attempt that’s not in your composed email - any inclusions, graphics, http links, etc.?
(If you email yourself can you reply to yourself with the VPN?)
Is the email address for your relative correct or somehow different in the received email? It may be rejecting it as an invalid address? Double-check the address in the TO: line when you hit reply.
Some email systems may block embedded graphics, or http links, etc. as spam.
Some email systems might be hostile to VPN’s (my educated guess) because spammers and others use those to hide their origins. Another possibility, when I had email through the local phone ISP, I could not send anything through a VPN because they only believed email clients coming from their own network. Outside, I had to use the web client. (Then came cellphones and such, and that restriction ended) Some servers use identified lists of VPN output points IP’s to block them.
So is the relative using the same email service? Perhaps your ISP provider is hostile to “From:Thisserver, TO: ThisServer” emails that don’t come from its company-provided network. Which would be weird in this day and age.
If you are sufficiently technical, is the VPN using the secured email (Port 993 or 587 usually) or unsecured (Port 110). Odds are the VPN blocks port 110. but then, all outbound email should be blocked.
Based on what’s been said, I’m guessing the problem is content related.
Lots of good references here, I’ll re-check. I’m also hoping I can get this to fail again to copy the error code.
Thanks for the reply.