Third Party eMail servers

[#1]

We are using our Plusnet broadband connection to access our business hosting company's email server and we have been having problems for the last month.

Thunderbird on Windows 10 wired to the router sometimes refuses to connect to the sending smtp server. Message

The message could not be sent because the connection to the outgoing server (smtp) mail.<domainname> was lost in the middle of the transaction, try again.

I asked the hosting provider  and they suggested changing the address to a direct server name but that didn’t help.

It looks like Plusnet is for some reason blocking us from accessing the server - though the problem is intermittent.

We can use the pop3 server to download the mail OK so it's not a password problem. No other internet facilities are affected (as far as we know).

The router is a Netgear N300 router - we can’t use the one provided as it doesn’t have the facilities we need.

The SMTP connection is using port 465 with SSL and MD5 Challenge-Response authentication.

We also get the same problem from our big office printer/scanner (wired) which uses email to send scans.

We also get the same problem when using an Apple iPhone mail system and also to other email server accounts (same provider) - these DO NOT get problems when connected to other networks. We haven't found anyone with a different provider who can try it yet.

The problem looks to be the same or similar to https://community.plus.net/t5/Tech-Help-Software-Hardware-etc/Third-party-email-problem/m-p/1587669#... and https://community.plus.net/t5/Email/Third-party-business-email-smtp-problems/td-p/1018477 but neither of those show solutions. Can you help please as it is getting very frustrating for our administrator

[#2]

Just for info, I connect to 2 totally separate non PN email servers using my PN internet connection without any issues whatsoever using Thundrbird (60.6.1 (32-bit)). Can send, can receive both pop and imap. I'd have thought if it was a PN problems you 'd have lots of people on here all jumping up and down. Just seems strange its intermittent. Do you actually get an email back saying it's undeliverable or just the error?

I know you say the PN router doesn't do what you want but have you tried using it for test purposes?  I normally use my own kit but its what I tend to do when problems surface. Start to break the problem down. What happens if you do a tracert to the email server when it fails to send? Does it fail at some point?

[#3]

Hi - thanks for the reply. Don't get me wrong, I'm not blaming Plusnet here just looking for help to locate the problem. If it is a problem with our host then I need some evidence to present to them as this is the only network for which it doesn't work.

Just the message comes back and the mail sticks in the outbox. No email reply.

I had considered the router but unfortunately the really basic one they sent is not even configurable to the IP range we use so it is not really an option, though I could try it with just the one device attached - I will try that later.

POP and IMAP are not a problem, only SMTP.

I will try a tracert (if I can find the app on Windows).

[#4]

Well I went in this afternoon and tried a few things.

No luck with tracert - SMTP servers (or at least ours) don't respond to ping so I wasn't getting anywhere. I did an online test of SMTP function and that was ok but I think it was using port 25.

I switched the router back to the one supplied but the results were inconclusive - perhaps because it was a quiet Saturday afternoon I wasn't getting many failures anyway but it failed at least once on this so I think it discounts that.

I tried reverting to non SSL port 587 and that seemed to work more often but not all the time.

I have also now seen a differnt error message

The message could not be sent because the connection to Outgoing server (SMTP) mail.<domainname> timed out.

I am wondering what to try next. As said, I doubt if it is a general Plusnet problem as many people would have complained but it looks like it may be a problem just with our line?