IMAP is Terribly Slow

IMAP is Terribly Slow

I have decided to move a bunch of mailboxes to Zoho and fired off imapsync to import emails.

From the same source mailbox (Amazon WorkMail), this is the speed I get transferring to:
- Zoho: 0.3 email/second
- Infomaniak: 14 email/second
- Gandi: 22 email/second

A transfer which should take no more than one hour has taken about 60 hours. I'm running the transfer from a datacenter (so I'm essentially not capped on bandwidth) in Europe, to Zoho EU. To exclude even that variable I've now started a backup (using offlineimap) of my Zoho mailbox and it's taking 3/4 seconds per email too.

By searching "slow IMAP" on this forum a ton of threads come up but support representatives seem to be consistently blaming this on the end user. I know for a fact this is not an issue with the source (same source to other destination goes pretty fast), nor with the intermediate node running the transfer (I used two different ones in DCs which are local to Zoho: Dublin and Amsterdam), so the problem must be in Zoho.

Is there some sort of deliberate throttling at play? Has anyone analysed this any deeper?

I'm pleased with Zoho's spam filtering so far and would love to not have to revert my migration.


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