SMTP Servers Blocked - Outgoing Mail will not send

[LPerkins]

I have an ongoing issue with my BT Full Fibre setup. I have several domains and associated email addresses which are hosted by one of the large web hosting companies.
When using Mac Mail on any of my Apple devices (MacBook Pro, Desktop, iPad, iPhone etc Mac Mail continually reports that the SMTP servers are offline.  Trying to change any server settings in Mac Mail is also almost impossible as the username and password can't be verified due them being offline. I have also tried using Thunderbird mail client with the same result.
Switching to 4g and sending from my phone or using webmail works fine proving the servers are not offline and all settings are correct i.e. port 465 SSL Password etc. and effectively means my hosting company is off the hook.
Switching the BT Home Hub off and leaving it off for sometime often results in the problem being resolved but only for a short time - sometimes a few hours, sometimes a few days.  I am making the assumption that the newly assigned IP address is solving the issue.  I have even managed to get a new router sent out and the problem was resolved for nearly two weeks!
Could it be that BT or the router is blocking my outgoing emails or blocking port 465?  It has proved impossible to get bt to look into this for me as they simply state they do not support 3rd party email addresses or it's my hosting company's problem blah blah blah which is of course a complete cop out. I have also ran multiple scans for malware and virus all with negative results.   As an aside my wife also has the same issues on her devices albeit with the same domains.  Any suggestions gratefully received before I throw the towel in and change to a different ISP? Many thanks

[Crimliar]

Just to make sure here.  If you are using the SMTP protocol the servers are often o port 587, if you are using SMTPS it's often port 465.

[Andyb911]

I have *exactly* the same issue.  Did you ever managed to resolve it?

[LPerkins]

I did solve it although I’m not sure it will help. I actually tracked it down to an issue with two network storage devices. These are Netgear NAS drives and it turns out they had been compromised causing them to send out suspect traffic (I’m still not sure what traffic they were sending) but it was causing the BT servers to block access to SMTP servers. Since the drives have been turned off for several weeks I’ve had no problem. To double check I did turn them on for a while and the problem returned within a few hours.  Perhaps you have something on your local network infected causing the same issue. I’m afraid I don’t know the name of the virus/malware in question as I’ve not yet found a way to scan the network drives. I found the issue by disconnecting devices for a few days at a time until I narrowed it down.  Took a couple of weeks though 🙂 Hope that helps