For InboundEmail connection problems, first try the Repair utility in Admin -> Repair -> Repair InboundEmail Accounts.
If that does not work, make sure the accounts can connect correctly via "Test Settings". That not working, try creating a brand-new test account and use the same settings.
That not working, file a bug

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I-E/Emails is going to get a whole SLEW of bug fixes in 4.5.0e. I have personally addressed 30+ since 4.5.0d. Here's hoping one that fixed recently isn't what is keeping you from using I-E in 4.5.0d.
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