Have been using Pegasus with both Fastmail <http://www.fastmail.fm> and GMX <http://www.gmx.com> via IMAP for years no problems. Mailstore on both IMAP servers is over 15k items each.

 GMX is a bit hard to find the configuration for standalone clients but can be done.

Do not like web mail screens and Pegasus handles IMAP much better than other clients (Mulberry / Alpine are IMAP champs - no html rendering).

if you search the forum there are a number of Yahoo issues raised and answered. 

I'm finding Pegasus's IMAP performance with Yahoo mail rather poor. (I'm using Yahoo mail's free, standard version, which supports IMAP.) The connections seem very fragile and unreliable. Pegasus frequently reports error advisories from the server, there are inconsistencies between what Pegasus shows and what's shown in Yahoo's web-client, folder refreshes fail, disconnections abound (though Pegasus doesn't call them that), and many of what Pegasus reports as server-errors cause it to completely crash. (These occurrences are quite frequent.) Are others experiencing the same?

I'm wondering if this is client related (i.e. poor handling of IMAP errors in general) or if Pegasus has more compatibility problems with Yahoo mail than it does with other IMAP servers. I'm also wondering if it could be related to any of the issues discussed here. That discussion is a few years old, so things may have changed since then, but some of the issues discussed there are: non-protocol handshaking by Yahoo and that Yahoo's IMAP is biased towards 3G use and doesn't like Wifi connections.

Would I get better IMAP reliability in Pegasus using a different provider? Pegasus does seem to work better for me with other IMAP servers, but my experience is not a fair comparison since my other email accounts are very low-volume usage compared to my Yahoo account.