Comment by p_l

Outlook when connected with exchange (which is probably the case, with corporate network email accounts connected) does not use SMTP nor IMAP, but Exchange RPC protocol, with underlying data model based on X.400 not SMTP. Can actually work pretty well but the implementation had been successfully eroded over last decade or more.

P.S. SMTP isn't well designed for slow and intermittent network protocols, it's designed so that you can bang it out on teletype by paying a grad student a twinkie and coffee and that should hopefully translate into simple implementation across different systems (only to relearn all the lessons of more complex ones, badly)