Likely a local issue judging from the absence of similar post on the forum. Check the Service status webpage and if further help is needed post back here with detail of the error message and email client involved.
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I've managed to find out a bit more about this. A new version of EM client was made available and users were invited to download. However, looking at their help forum it's been a nightmare. I tried disabling my anti-virus and that worked for a while, all folders and mail was synced successfully. I then tried just disabling email function on the AV, and that was OK, but then I started to get SMTP errors. Managed to get through to their helpdesk and they said they were working on a fix for the AV problem, it was known.
Think I will just use webmail until they have a fix. I checked the IMAP and SMTP account settings and they all appear to be OK.
I have reverted back to V7 of EM Client, 8 was a joke. I have noticed that the account settings for SMTP are shown as Use Port 25, and use SSL/TLS if available although the VM instructions are Port 465.
Everything is working OK, so assume that doesn't matter.