Using classic Outlook for Windows in business environments
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I still see no issue having both NEW or OLD versions running for those who wish to use either.
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There is no issue running classic and new outlook at the same time, or even Thunderbird. eM client, or another email client at the same time.
I usually have both outlooks running and occasionally eM at the same time - all with most of the same email accounts in them. On rarer occasions also had Thunderbird open. The only issue would be if you had POP accounts (a lock is put on the mailbox while the client is downloading) but new outlook does not support POP.