You can solve this problem by increasing the swapper.
Highly unlikely. The swapper will expand itself, up to the limit of disk space minus the first number on the swap line, and it will shrink again, down to the defined size, but that causes a lot of overhead, which is why you want to define a larger minimum size. In my experience, the swapper will never be used, if you have 4 GB of memory, and it would be very rare if it ever gets used with more than 2 GB of memory. If you have a small memory system, increasing the swap size may help (but not likely with this problem).
On the other hand, changing VIRTUALADDRESSLIMIT to, at least 2560, in CONFIG.SYS, will make more shared memory space available, but it only expands the upper shared memory space, so you need to use all methods available to use upper shared memory, rather than lower shared memory. Doing that moves some things out of low shared memory, leaving more room for those things that can't use upper shared memory (PMMail is one of them). The max for VIRTUALADDRESSLIMIT is 3072, but some systems don't work with 3072, so I suggest using 2560. This is more of an art, than a science.
-Doug Bissett: Are you really sure that you do not have this error C000005, in your popuplog.os2?
It happens sometimes. PMMail even gets that sometimes (even when using EXCEPTQ, which prevents entries in POPUPLOG.OS2), but it is usually not the cause of a problem, it is a result. The information in a TRP file is more likely to give good information, if you know how to read it, and know what to do with the information. PMMail also has excellent logging, if the user enables it properly.
The bare hooks are there for IMAP support -- it would be nice to have a volunteer add and support that feature.
There was a volunteer, at the 2006 WarpStock, but he had to back out, and nobody else has stepped up to do it. I suspect that the current PMMail needs some serious updates, before IMAP will work effectively. Most of the really bad stuff has been fixed (we are not sure how the program ever worked, before VOICE took over development), and work arounds have been developed to support modern mail servers (using STunnel for secure access), but the container (for display) is being pushed to the limit, so that whole thing needs to be reworked. It is also using an XML package, to support the indexing, and that has proven to be very slow (apparently by design). Something should be done about that. Unfortunately, going back to the old method of indexing, is not compatible with supporting languages other than English, so that isn't going to happen.
There was, at one time, some discussion about actually supporting HTML e-mail, and that could be done properly, if somebody can, and wants, to do it. Currently, PMMail simply hands the message to a browser, but responding with HTML is not easy.
It could be a lifetime project, for someone. It seems that SeaMonkey, and Thunderbird, are probably dead on OS/2 (following Firefox), so something needs to be done, if we are to have an e-mail program. PMMail, is the only other e-mail program that is still (almost) active, that I know about.