i'd be more apt to think squirrelmail could handle it considering the probs hivemail has had in the past (according to some older posts anyhow). another WHT'er put me on to this site where you can buy nice skins for squirellmail: http://nutsmail.com/
If you need just a webmail, never go with HiveMail, it is more than just a webmail.
Squirrel Mail or any other web mail works better as it won't have to process emails, they just show emails processes by email server like qmail, exim, etc... In the case of HiveMail, all mails are served to a PHP script, then it have to read the mail and handle the mail storage.
For reliable mail servers, better go for qmail based mail server with any webmail.
Socketmail, i don't think it will be much different from HiveMail. Both use PHP program to process emails. That is your email server accept all emails (spam and directory attack) and PHP script have to process each emails and store in MySQL or File System.
In real mail server, if recipient is not available in the server, it will be rejected in inital SMTP process itself.
atmail is very costly. It seems that too use same catch all processing, instead of PHP, it use Perl. Since it is very costly, those who buy it are ready to invest good amount on server too :-)
I have checked some version of AtMail, that use same technology as HiveMail, that is pipe from real mail server, which do catch-all, to a perl script, that process all emails.
Do not know if it will be different if we get atmail email server. Or it is just a paid replacement for free mail serves like exim, qmail, sendmail, etc...
If HiveMail increase price to $2700, only few will buy it and those who buy will able to spend $500 per month on hosting it. Many of HiveMail installations run on shared server. So i think HiveMail and AtMail are almost same.
Atmail claim to have their own email server package. If you click on their clients page, there is a list of clientele and Hostway utilizes their multi cpu/server module. Check it out. I like Hivemail but it the company went passive.
Socketmail is abetter choice. (IMHO).
It has a spam-mail guard, that blocks unwanted mail and send the sender a verification image email to confirm they send the socketmail user an email then delivers the mail.
Socketmail works on php (lite edition) like Hivemail. But uses a pipe system to deliver mail quick and easy. It also features email n file sharing between email users of same domain. Also features WAP/GPRS addon.
Socketmail Pro is like a webmail + pop3 and smtp support. Meanign you can keep acopy of the mail in your webmail and get a copy in outlook express. Also features WAP/GPRS addon. Integrates with EXIM (cpanel servers) or Qmail or other Email servers - meaning its pretty much the same as squirrerl n stuff...(in the integration way) which means u can have ur emails scarnned vby CLAMAV or any other integrated antivirus filter + the antispam filters you install with your email system (MTA).
As for Hivemail ...used them, been there, left there...HIvemail is a mini socketmail...which is good too Its got nice customizable skins which is pretty cool BUT the last hivemail udpate was over a year ago and its almost a dead going service (actually i think it is dead)...
Socketmails last update was 4 days ago and they make new updates every 4 months ..
ATMAIL..dont know them so wont comment on them.
This is my suggestion
1. Socketmail Pro
2. Socketmail Lite
4. Atmail (since i dont know it at all)
5. SquirrelMail with coding module integration to provide outside signups (required Squirrel Customization) a bit costly too