Server error if sending email with attachments, but only from one laptop...

OK, I have what seems to be a pretty weird issue, and would love a hand... ISSUE: On 1 laptop on my home network, I get error messages from the Optus email servers if I have an attachment with the email (occasionally I DON'T get the error, but the attachment is received at the other end corrupted). DETAILS: 1. As mentioned above, it's only on one of the three latops on my home network. Neither of the other 2 have any issues sending emails 'normally'. 2. Also as mentioned above, sometimes the message does get through, but the file is always corrupted - I've tested different file types (pdf, word etc). Always the same. I've even tested small and large files... IF the server doesn't throw back an error, the attachment gets to the other end corrupted 3. The machine in question is a very new Alienware 17R5 laptop running Windows 10, and the O365 Outlook Client *NOTE* Client as in actual client - i have an O365 subscription and installed the whole office suite on the laptop- including outlook. 4. When I first set up email, i did not have the issue. 5. I did all the usual things like disable my AV (bitdefender) and even uninstalled Bitdefender in desperation to check. 6. When that didn't work, i took the drastic step of doing a Windows reset. Restored the PC back to clean, and started all over again. 7. When I did that, I reinstalled Outlook, setup email again for Optus. It worked! 8. Within a few days the server errors have come back. 9. I disabled, then uninstalled Bitdefender again (just in case). Still getting errors. 10. I experimented by adding some of my various other email accounts to the Outlook client (a gmail account, and an account). When I send from either of these accounts i DO NOT get an issue. Attachments send fine. 11. If I get the error an walk away from the machine, eventually when I come back it's usually sent. So at some point the server accepts it on a retry. Without fail, when it DOES send, the attachment comes out corrupt at the other end... 11. I then did another experiment to narrow it down. I installed Thunderbird to help confirm it really wasn't Outlook. Added my optus account. Same behaviour. Emails send. Emails with attachments throw up a server error. 12. I added my account to my wife's laptop - (still Windows 10 with Bitdefender and O365 Outlook Client installed). I do NOT have the issue. No errors when sending from my account. No corrupt attachments) 13. Finally, I added my wife's optus email account to my machine to test. Same errors as for my account. footnote - I also have my optus email on my phone. no issues at all there... So... What I know: i. It's only on one machine ii. It's only when I send attachments iii. It's only when i send through my optus email account (my main one)... I'm lost. If it was a legit server-side error why isn't it happening from any machine on my home network? Why is it just attachments? If it's a clean install and works, then fails, why am I getting SERVER error messages back? Error message from Thunderbird client "The message could not be sent because the connection to Outgoing server (SMTP) was lost in the middle of the transaction. Try again." Error message from Outlook client can vary - most recent one when I tried when writing this was "reported error (0x8004210B): 'The operation timed out waiting for a response from the sending (SMTP) server. If you continue to receive this message, contact your server administrator or ISP' MY WILD GUESS: i THINK it could be something screwy with the laptop. Given it's only on this laptop, makes sense. But why are the errors seemingly server-side related? Why only attachments, not text only emails? Why only this one email provider (that I know of) in my Optus accounts? And why, if it's the laptop, did it work initially on the laptop for months, then start the issue. Then when I rebuilt it worked for a few days before it happened again? That could mean a driver/OS update is the culprit, but seems again to be at odds with the fact other email accounts work fine. I'm so confused on this one. 20 years in the IT industry (admittedly i don't play with the tech much these days), but started out on help desks and engineering. I'm pretty decent at troubleshooting. This is killing me. Anyone out there got any ideas?

Re: Server error if sending email with attachments, but only from one laptop...


Yes it would seem like something quirky with that Alienware. Have you tried that machine on another network such as tethering from a phone?

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