If I open eM Client and retrieve email from my main “metrocast” account, those emails disappear from the inbox after I open OE Classic and those emails are retrieved via their software.
This behaviour you describe seems to reflect that the account is set as IMAP in eM Client while in the other email client it’s set as POP3. It means that right after you open the OE Classic, the mail is downloaded and deleted from the server and this change is immediately reflected in eM Client by disappearing of the message.
To solve this, you can either set the OE to keep messages on the server after downloading with POP3 protocol or set the account in OE as IMAP.
How do I do either one - set the OE to keep messages on the server or set the account in OE to IMAP?