I have been trying out EM Client Version 7 for the past couple of weeks on my computer as well as my wifes computer both Windows 10.
Three times now the client has decided on both computers at different times to send multiple copies of the same email with the attachment to the receiver based on the send receive time on the client. In other words automatically check email = automatically resend email with attachment.
In one case the client did it over 50 times before it was noticed.
The only way to stop it from happening is to uninstall the program and delete all of the info in Roaming/User/app data/emclient.
Reinstalling the program makes everything ok for a few days or a week.
To be 100% sure that it is not a virus etc. I reinstalled WIN 10 from scratch and just installed EMClient. Three days later, it did it again.
It’s happening only when there are attachments, which can be anywhere from a few hundred K to close to 10 MB. emClient just keeps sending copies, although some of them are good at the recipient’s end.
We are using our own email server (standard Plesk VPS).
I vaguely remember a case some time back where the server was not correctly closing the connection, so the client was resending the message again and again. I don’t find it on this forum so it might have been Thunderbird.
Hopefully, someone might have had a similar experience with the same server you are using, and can comment. In the mean time, it probably would be quicker for you to open a support ticket with eM Client. who should be able to assist you further.
Since we are using TLS i think that there would be a retry if packets went missing (that’s mainly why i use TLS), so most likely it’s a timeout problem and emClient doesn’t wait long enough. Just a guess.
This is clearly related to a slow ISP but it would probably be more trouble than it’s worth to track down.