Outgoing emails inexplicably fail to send

[ICL]

I use Outlook for Mac as my email client; my wife uses Outlook on her laptop PC.  We have been BT broadband users from the beginning, but BT has not yet reached us with FTP so we are presently signed up with Giganet/Cuckoo Fibre, anticipating changing back when BT FTP reaches us. We are still premium BT email users and still have some BT broadband which we do not normally use, being very basic and slow.

We both receive BT emails without any problem. However, although the day will normally start with BT emails being sent instantly, we both at some point, pretty much every day, have a problem with emails not being sent and error messages either "The connection to the server failed or was dropped" or "Invalid user ID or password from [IP address]". The problem remains with emails refusing to leave the Outbox, but incoming emails continuing to be received.

If we disconnect from Giganet/Cuckoo Fibre and connect to BT Broadband, without making any changes, the emails leave the Outbox.

Any learned ideas?

[licquorice]

Probably a faulty BT SMTP server.

Changing from one provider to another will result in a different IP address and thus different server.

[ICL]

But why is there a faulty BT SMTP server every day, whether I use VPN or not, and only on access via Giganet FTP and never on access via BT Broadband?

[licquorice]

Because you are probably picking up the same faulty server every time with Giganet, but a good one with BT. The routings will be totally different.