Scalability of Dovecot in the Cloud

[KT Walrus]

Does anyone have any idea of how many IMAP connections a single cloud VM (4 vCores at 2.4GHz, 30GB RAM, local SSD storage - non-RAID) can be expected to handle in production. The mailboxes are fairly small (average 5MB total - 50MB max, as I don’t store attachments in Dovecot expect those saved through IMAP in the Sent/Drafts folders) and each user will probably have an average of 2 devices that have the mail clients configured to access each mailbox.

Can such a server handle 100,000 mailboxes (200,000 devices/clients)? Or is it more like 10,000? Or, even smaller?

I can scale the cloud VM up to 32 vCores and 240GB RAM (at 8 times the price) or split the mailboxes onto multiple VMs. The VM will also be running LMTP and other Dovecot services (I don’t plan on supporting POP3 at this time). The mailboxes will be sync’d to a backup VM running Dovecot for high availability so has some load from this background activity. LMTP will not be that high a load, I think, since most messages will be delivered by at night. But, clients will have IMAP connections 24/7.

Just trying to get an idea of the cost of running a potentially huge/growing mail service in the cloud… I’m going to have to support around a million mailboxes before the site will generate significant revenue to support operations.

Kevin