Ok - I've worked out that its not a problem with my iPhone. I've tried putting that account on a different iPhone and the same crash happens (using pop, imap, securemail & normal). Strange - crash file below if anyone can read these things....
Incident Identifier: 7E4CAF41-ED98-4271-B5DD-E58A9A05C55E
CrashReporter Key: 38377d2d0ac5fb6ec0cdc83da0479344093a2a33
Process: MobileMail [57]
Path: /Applications/MobileMail.app/MobileMail
Identifier: MobileMail
Version: ??? (???)
Code Type: ARM (Native)
Parent Process: launchd [1]
Date/Time: 2009-10-08 07:43:40.870 +1100
OS Version: iPhone OS 3.1 (7C144)
Report Version: 104
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x034b4000
Crashed Thread: 5
Thread 0:
0 libSystem.B.dylib 0x3080c4b8 0x3080b000 + 5304
1 libSystem.B.dylib 0x3080f094 0x3080b000 + 16532
2 CoreFoundation 0x3147a002 0x31423000 + 356354
3 CoreFoundation 0x31479c18 0x31423000 + 355352
4 GraphicsServices 0x325323a0 0x3252e000 + 17312
5 UIKit 0x3314ec28 0x3314b000 + 15400
6 UIKit 0x3314d228 0x3314b000 + 8744
7 MobileMail 0x000237ac 0x1000 + 141228
8 MobileMail 0x000026e8 0x1000 + 5864
Thread 1:
0 libSystem.B.dylib 0x3080c4b8 0x3080b000 + 5304
1 libSystem.B.dylib 0x3080f094 0x3080b000 + 16532
2 CoreFoundation 0x3147a002 0x31423000 + 356354
3 CoreFoundation 0x31479c18 0x31423000 + 355352
4 WebCore 0x31e196f0 0x31d95000 + 542448
5 libSystem.B.dylib 0x308367b0 0x3080b000 + 178096
Thread 2:
0 libSystem.B.dylib 0x3080c4b8 0x3080b000 + 5304
1 libSystem.B.dylib 0x3080f094 0x3080b000 + 16532
2 CoreFoundation 0x3147a002 0x31423000 + 356354
3 CoreFoundation 0x31479c18 0x31423000 + 355352
4 Foundation 0x31386960 0x31328000 + 387424
5 Foundation 0x313868ae 0x31328000 + 387246
6 MobileMail 0x0004fcf8 0x1000 + 322808
7 Foundation 0x31329d0e 0x31328000 + 7438
8 libSystem.B.dylib 0x308367b0 0x3080b000 + 178096
Thread 3:
0 libSystem.B.dylib 0x3080c4b8 0x3080b000 + 5304
1 libSystem.B.dylib 0x3080f094 0x3080b000 + 16532
2 CoreFoundation 0x3147a002 0x31423000 + 356354
3 CoreFoundation 0x31479c18 0x31423000 + 355352
4 Foundation 0x31386960 0x31328000 + 387424
5 Foundation 0x313868ae 0x31328000 + 387246
6 AppSupport 0x32500596 0x324f7000 + 38294
7 Foundation 0x31329d0e 0x31328000 + 7438
8 libSystem.B.dylib 0x308367b0 0x3080b000 + 178096
Thread 4:
0 libSystem.B.dylib 0x308312f0 0x3080b000 + 156400
1 CoreFoundation 0x314437e2 0x31423000 + 133090
2 libSystem.B.dylib 0x308367b0 0x3080b000 + 178096
Thread 5 Crashed:
0 libSystem.B.dylib 0x30817c1a 0x3080b000 + 52250
1 MIME 0x30739b42 0x3072f000 + 43842
2 MIME 0x30739c34 0x3072f000 + 44084
3 MIME 0x30746b92 0x3072f000 + 97170
4 MIME 0x3074722a 0x3072f000 + 98858
5 MIME 0x30746f56 0x3072f000 + 98134
6 MIME 0x30747430 0x3072f000 + 99376
7 MIME 0x3074ca38 0x3072f000 + 121400
8 MIME 0x3074c906 0x3072f000 + 121094
9 MIME 0x3074c954 0x3072f000 + 121172
10 MIME 0x3073f020 0x3072f000 + 65568
11 MessageUI 0x32fbf584 0x32f99000 + 157060
12 MobileMail 0x0004ddf0 0x1000 + 314864
13 MobileMail 0x0004e50c 0x1000 + 316684
14 CoreFoundation 0x314348dc 0x31423000 + 71900
15 CoreFoundation 0x31488574 0x31423000 + 415092
16 Message 0x3187c24c 0x31879000 + 12876
17 Message 0x31888010 0x31879000 + 61456
18 Foundation 0x3137bac6 0x31328000 + 342726
19 Foundation 0x31329d0e 0x31328000 + 7438
20 libSystem.B.dylib 0x308367b0 0x3080b000 + 178096
Thread 6:
0 libSystem.B.dylib 0x3080c52c 0x3080b000 + 5420
1 libSystem.B.dylib 0x3083f5d4 0x3080b000 + 214484
2 libSystem.B.dylib 0x3083e3ec 0x3080b000 + 209900
3 libSystem.B.dylib 0x3083f5bc 0x3080b000 + 214460
4 Foundation 0x3138643e 0x31328000 + 386110
5 Foundation 0x31385ff0 0x31328000 + 385008
6 MIME 0x30749ce4 0x3072f000 + 109796
7 Message 0x31887f02 0x31879000 + 61186
8 Foundation 0x3137bac6 0x31328000 + 342726
9 Foundation 0x31329d0e 0x31328000 + 7438
10 libSystem.B.dylib 0x308367b0 0x3080b000 + 178096
Thread 5 crashed with ARM Thread State:
r0: 0x034b4000 r1: 0x0000002d r2: 0xfd399d13 r3: 0x00000000
r4: 0x034a9425 r5: 0x33ee8b64 r6: 0x0084dd13 r7: 0x04080558
r8: 0x04524360 r9: 0x001fc098 r10: 0x00000024 r11: 0x001d1320
ip: 0x3809f30c sp: 0x04080534 lr: 0x30739b49 pc: 0x30817c1a
cpsr: 0xa0000030