topic Re: Internal server error when sending an email in Email

Internal server error when sending an email

remikz — Mon, 06 Oct 2025 18:35:28 GMT

I'm getting an internal server error when trying to send an email using Thunderbird.

It's been fine for months, but I haven't been able to send an email in the past few day,

Re: Internal server error when sending an email

jab1 — Mon, 06 Oct 2025 18:39:27 GMT

Which email brand?

Re: Internal server error when sending an email

Townman — Mon, 06 Oct 2025 19:08:03 GMT

What error code?

On network or off network?

What platform?

What AV software?

Re: Internal server error when sending an email

MRChambers — Mon, 06 Oct 2025 19:27:13 GMT

I've had the same today (4.3.2 Internal Server Error) on force9 whilst on Plusnet network without authentication. I've solved it by switching to authentication. (AV is Kaspersky but eliminated that as a cause).

Re: Internal server error when sending an email

Townman — Tue, 07 Oct 2025 14:54:32 GMT

Do you have a static IP address?

Re: Internal server error when sending an email

MRChambers — Tue, 07 Oct 2025 16:19:06 GMT

Yes, I have had a static IP for many many years. (Was a necessity at the time for dialup modem access to my office network, no longer essential)

Re: Internal server error when sending an email

MrZeno — Tue, 07 Oct 2025 17:03:25 GMT

I'm having the same issue. Every attempt at sending an email returns an "Internal Server Error". I can receive emails without an issue.

Force9 account, Thunderbird client, and not using a static ip, Windows 10.

Re: Internal server error when sending an email

remikz — Tue, 07 Oct 2025 19:49:45 GMT

For me this is: Force9 account, Thunderbird client, fixed IP, Windows 10.

Re: Internal server error when sending an email

Katie_B — Wed, 08 Oct 2025 12:23:49 GMT

Hi @MRChambers @MrZeno @remikz

Could you please try sending an email via webmail and let me know if you see a more specific error message?

Speak soon,

Katie

Re: Internal server error when sending an email

MrZeno — Wed, 08 Oct 2025 16:29:09 GMT

Using Firefox I successfully sent an email from the force9 webmail to my gmail account ok, no error.

This suggests a problem with Thunderbird configuration? If so then this is odd how it has suddenly begun after years of no issues.

Thank you.

Re: Internal server error when sending an email

Townman — Wed, 08 Oct 2025 16:32:31 GMT

@Katie_B

Sending via webmail will totally side step the issue here.

The summary of this issue (Server error 4.3.2) gathered elsewhere suggests the following profile:

SMTP: relay.force9.net
AUTHENTICATION: None
Possibly having a static IP address might be a factor

Setting authentication will eliminate the issue ... unless the user's (whatever) is on the black list.

@plusnettony has requested (via the new forum team) that examples of the 4.3.2 server issue be gathered and added to the open incident.

BTW - your 'bravery' of stepping in to the complexity of the email space is welcomed and admired!

Re: Internal server error when sending an email

Townman — Wed, 08 Oct 2025 16:32:14 GMT

@MrZeno wrote:

This suggests a problem with Thunderbird configuration? If so then this is odd how it has suddenly begun after years of no issues.

This is not a client configuration issue in its self. Your reflection suggests that your configuration has never been set up to require authentication ... which up to a point is fine, so long as the device is never on a network which is not PlusNet's.

Re: Internal server error when sending an email

MrZeno — Wed, 08 Oct 2025 17:00:10 GMT

I've done what you said about changing the authentication to none, and it didn't work. So, I've gone through every authentication and connection security options and either I get a timeout or internal sever error.

Re: Internal server error when sending an email

jeffh — Wed, 08 Oct 2025 17:05:47 GMT

Since start of october I have had exactly the same Internal server error problem with sending SMTP port 25. Sending works using Thunderbird under Linux on SSL/TLS port 465. I have tried copying the setup from Thunderbird into Pegasus Mail running in Wine under Linux but it won't even start the dialog with the server. I suspect this is because Pegasus is an ancient Win3.1 program and does not support the current protocol, it is missing some of the setup options in Thunderbird. It would be really handy if port 25 worked like it used to because Pegasus is my daily driver for mail.

Re: Internal server error when sending an email

Townman — Wed, 08 Oct 2025 17:22:54 GMT

Email brand is vitally important in these discussions. Internal server error has been encountered on Force9 and free-online but not Plusnet email brands.

It does not matter which SMTP port is used.

It is only encountered where the connection attempt is NOT AUTNETICATED.

In terms of being compliant to recognised standards, neither port 25 or 465 should be used.

The correct settings for the SMTP server are...

Port: 587
Authentication: same as incoming server
Encryption: STARTTLS (or TLS on some non-standard email clients)

The actual server is that applicable to the email brand.

In passing, it is of some interest to know if those users encountering the issue have a static IP address.

Re: Internal server error when sending an email

Townman — Wed, 08 Oct 2025 17:24:14 GMT

@MrZeno wrote:

I've done what you said about changing the authentication to none, and it didn't work.

I do not believe that I suggested that - the authentication should be set to the same as the incoming server.

Re: Internal server error when sending an email

Townman — Mon, 13 Oct 2025 10:44:21 GMT

@remikz / @jeffh / @MrZeno / @MRChambers

Some of you have mentioned "Internal Server Error" but not specified which error code. There are potentially two distinct issues here, which is why precise detail is always important.

Internal error 4.3.2 relates to the absence of authentication credentials on the SMTP server for relay.force9.net - set authentication to use the inbound server's credentials
Internal error 550 suggests that your credentials have been the object of service abuse
The second might be 'disclosed' when the former is fixed

Sender Domain Rejected (black listed by local policy)

This issue can report as server error 550. It typically arises where a user's email address has been abused for the purposes of sending spam emails. It has nothing to do with server settings etc.

This requires intervention by staff to determine the cause of the problem and to issue advice to check your system for viruses and malware. Only after your system(s) have been affirmed to be clean can the black listing be lifted. As a precaution, you should change the password on the affected email address.

Add your details to this thread - Cannot Send Emails and send a PM to @Leanne_T

Re: Internal server error when sending an email

remikz — Thu, 30 Oct 2025 08:22:54 GMT

Setting authentication resolved the issue for me.

Previously you didn't have to do this when connected to Plus.Net but it seems you do now.

Re: Internal server error when sending an email

Townman — Thu, 30 Oct 2025 08:32:22 GMT

@remikz

The requirement for this going forward is in the migration FAQ.

@dave / @plusnettony

Can the requirement for SMTP authentication be included explicitly in the migration communications please.