There's no change at TalkTalk's end as regards settings or configuration.
Perhaps you're not aware of new policies at Yahoo Mail and Gmail regarding spam? Gmail are reserving the right to dump any mail in Spam folders if more than 3 in 1000 messages sent are marked by Gmail recipients as spam. So in enacting that policy at Gmail any mail message can be dumped into spam without checking if it's genuine or not. TalkTalk have no control over these new policies.
All TalkTalk Mail is sent fully authenticated so any mail direct to any other mail service will be seen as genuine and should be delivered to the recipient's Inbox. Those recipients that have contacted you should be asked to look in their spam folder and if your messages are there to select and mark as Not spam. This will tell the mail service not to drop your mail in spam but to deliver future mail to the Inbox as normal.
As you seem to be using TalkTalk Mail for business purposes you should also be aware that TalkTalk Mail usage is for home use and therefore there are reasonable limits on sending. If you want to exceed normal residential email usage then perhaps you should consider a dedicated business mail service? Just a thought.
