On the ISP-Tech list in April, DH queried,

“Would someone please explain the benefit of providing IMAP support for email? I really would like to understand the reason why so many ISPs seem to be offering this.”

A number of respondents explained the essential differences between POP3 and IMAP:

[BS offered] “POP3 sucks mail off the mail server and stores it locally: the mail server is simply a way-station on the way to the user’s hard disk. IMAP keeps mail on the mail server. Just like a file server provides centralized file storage, an IMAP server provides centralized mail storage.”

[AC added] “One of the significant benefits of IMAP is the ability to download the headers only, thus saving valuable time and bandwidth on emails you don’t want to read for whatever reason.”

[GS recalled] “We’ve used IMAP instead of POP3 where a mailbox absolutely needs to be tended several times a day, regardless of who tends it. If it’s POP3, then it ends up stuck on somebody’s desk, so it might get missed if they’re out. If it’s IMAP, it stays on the server until any team member sees it and either deals with the issue and deletes the email from the server, or leaves it there for another team member.”

Others suggested that any decision between the two should depend on the makeup of your customer base:

[BS noted] “To your average ISP targeting the home user, POP3 is preferred: less maintenance, fewer headaches, and no need to worry about mailbox storage. To a managed services provider, on the other hand, IMAP is great. You can provide all sorts of added services, for added fees, of course: centralized management, remote access to mail, web access to saved mail, mailbox backups, client roaming, etc.”

[JL agreed] “As an ISP targeting residential accounts, POP3 is definitely better. Many of our subscribers simply check the little box in their email client which says, ‘leave mail on server,’ and last guy to read the messages deletes them. Works great. Looks to me like IMAP is a nice alternative, but only for an ISP that can charge significantly more for the server storage and maintenance overhead.”

[JM countered] “If you do any work with small companies where you operate their email service, I’d consider IMAP essential. Trying to share mailboxes with POP3 can be tricky. There are always users who don’t realize that eventually someone must delete those messages from the server, and you end up fielding calls when their disk quota runs out. IMAP is really nice for sharing mailboxes.”

Still others pointed out that offering IMAP could be a great way to stand out from the competition:

[JM advised] “Like a lot of other service offerings, if an ISP offers something like IMAP, this just differentiates them from the next ISP. Since only a small percentage of users are going to take advantage of it anyway, it’s not likely to cost you very much.”

[DL agreed] “95 percent of your users won’t know there’s anything better than POP3 for ISP mail. But a lot of your users would probably love IMAP if they discovered it, and a large number would come back from Hotmail, Yahoo Mail, etc., if they had the features of IMAP available.”

[SS added] “I like giving people a choice. Some of my customers choose POP3. Most don’t care, and I tell them about the benefits of IMAP and get them set up using it. Some get excited about having an IMAP server available. Personally, I could not live without IMAP.”

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