Outbound e-mails rejected by receiving servers since recent Plusnet major e-mail outage

[#1]

For outgoing work e-mails sent from home, I use the Plusnet SMTP server, but with a work non-Plusnet e-mail address as my sender / reply address.

In the past there have been a (very) few issues with some receiving servers rejecting e-mails because the Plusnet server does not match the e-mail address I am using.  Whitelisting me at the receiving end has always solved that. 

Since the recent major Plusnet e-mail outage these rejections have become a recurring theme and many receiving servers which previously accepted my e-mails are now rejecting them.  Even whitelisting me at the receiving end does not always seem to solve the problem.   It's almost as though one or more of the Plusnet servers may have got themselves onto a spam blacklist

Can anyone explain why this change has happened and if there is a solution ??   

[#2]

For outgoing work e-mails sent from home, I use the Plusnet SMTP server, but with a work non-Plusnet e-mail address as my sender / reply address.

That's really not a good idea. As you say, you have in the past had a few rejections. The SPF record for your work domain will not specify the PN servers as authorised senders. SPF is being used more and more to help detect spam, some servers will just reject if no SPF match whereas other will use it as part of the spam score assessment.

Two possible solutions:-

1) Send the email via the work server

2) If you have access to/control of the work domain DNS, then add the PN servers to its SPF record

[#3]

Thanks for the suggested workarounds, neither of which is very practical for various reasons. I am trying to work out what has changed recently to cause the problem, as that may give some clue how to actually solve it and get back to where things were.

[#4]

I'm afraid that without an SPF record that specifies the originating server as a valid sender, the situation will likely deteriorate as more receiving servers make use of it.

[#5]

One needs to ask why are you seeking to send work emails via Plusnet's SMPT servers and not your work's email servers?

It is not normal to seek to route outbound email via a third party SMTP server, it just adds complications.

[#6]

I have been doing it this way for 25+ years. I was originally told that I could not use the outbound work server when not connected to the work network, and such a VPN connection was often problematic when travelling in areas with slow internet connections. It all worked pretty much seamlessly (with odd exceptions) until the massive Plusnet e-mail blow up a month or two back.

[#7]

I suggest that advice is markedly dated.  It was often the case that SMTP connections were not set up with user authentication settings (may be because it was easier?).  When computers were nailed to desks and by implication a known network, this was not that much of a problem, however when computing platforms became mobile, the necessity to change SMTP severs for each new network connection, such practices became impractical.

For a long time, the de-rigour approach has been to always use the SMTP server associated with the email domain, using authenticated connections on port 587.  If encryption is used it should be STARTTLS.

As for things which worked, no longer working, one should not confuse coincidence and cause.  @MisterW has pointed to the most likely cause of your issues - Plusnet relay servers are not authorised SMTP servers for your work's email domain.  You could look at exploring that for yourself using nslookup for your employer's domain.

In a cmd window, type the following commands at the > prompt...

C:\Users\you>nslookup
Default Server:  UnKnown
Address:  192.168.1.254

> set type=txt
> yourdomainname.co.uk
Server:  UnKnown
Address:  192.168.1.254

Non-authoritative answer:
yourdomainname.co.uk text =

        "v=spf1 a mx include:_spf-internal.plus.net include:_spf-internal2.plus.net ~all"

If there is a spf1 record, please advise of the character preceding the "all".  That will define the level of enforcement (if any) set by your employer.  If there is a stringent SPF setting, then email receivers applying strict source verification will quite rightly reject emails submitted by a non-approved SMTP server.  Such is the right and proper measures wise domain owners take to ensure that email spoofing on their domains cannot happen.  It is not beyond the realms of possibility that your employer has implemented more stringent SPF measures in the same timeframe as Plusnet had email service issues.

[#8]

Mail servers will inevitably get tagged as suspect periodically. This will affect the spam scorevbut providing the sender has,a,valid SPF it will likely not be enough to reject the email as,spam. With an invalid SPF. The spam score will be higher and in some circumstances enough to classed as,spam

[#9]

Thanks for all the answers. Townman's explanation of a coincidence of timing sounds highly plausible, given that I believe we have recently been reviewing our IT security. Having been pointed in that direction, I will take the SPF issue up with our IT people and report back .