e-mail from desktop rejected as spam

[HumbleBee]

Some mail sent from my desktop client using my btconnect.com account is being rejected as spam.  I get returned messages including things like:-

Remote Server returned '554 5.2.0 lmCX1r00Y2wdUrh01mCYtM Spam content found'

If I send an identical message from the same account from the on-line instance of Office365 then the message is accepted.

This is a recent problem (a month or so), but getting more frequent.
It happens with mail to specific domains (e.g. Tesco.net), not all domains.

The implication is that something at the recepient's mail host is set to reject mail that comes from a desktop client even though the account is entirely genuine.

Set-up:
Account: <me>@btconnect.com
Client: Thunderbird

Protocol: SMTP

Port: 557

Connection Security: STARTTLS

(None of the above has changed since the time before the problem started.)

What can be done to prevent rejection?

[markp · Accepted solution]

Hi HumbleBee,

what the issue could be is that the receiving servers is checking the IP the mail is coming from (your connections IP)

and it is seeing it on a blacklist.

Have you tried rebooting your router to get a different IP?

Is the issue still there after a reboot?

Markp

[HumbleBee]

Yes, I'd wondered about that.  I'll reset, test and report back.

(The first test, leaving the Modem off for five minutes before restarting, came back with the same IP address.)

[HumbleBee]

Markp,

Yes, rebooting the router and obtaining a new IP address (actually switching it off for several hours to avoid obtaining the same IP address again) did the trick.

So far so good.

In the process of investigating, I discovered both the old and new IP addresses on a Spamhaus "warning" list and on at least one other list - thus BT is allocating IP addresses with dubious history to its business customers.