MessageFramework on CocoaDev lists various possibilities,
"Sending emails from Cocoa" also lists several frameworks, including Pantomime, MailCore and EdMessage (the site for this seems to be down, but there is a mirror on github, which has also been modified to compile for 10.4, as well as 10.5 - so should work on the iPhone)
Example code using Pantomime (from the above blog-post):
CWMessage *message = [[CWMessage alloc] init];
CWInternetAddress *address;
address = [[CWInternetAddress alloc] initWithString:@"[email protected]"];
[message setFrom:address];
[address release];
address = [[CWInternetAddress alloc] initWithString:@"[email protected]"];
[address setType:PantomimeToRecipient];
[message addRecipient:address];
[address release];
[message setSubject:@"test"];
[message setContentType: @"text/plain"];
[message setContentTransferEncoding: PantomimeEncodingNone];
[message setCharset: @"us-ascii"];
[message setContent: [@"This is a simple content." dataUsingEncoding: NSASCIIStringEncoding]];
smtp = [[CWSMTP alloc] initWithName:@"smtp.gmail.com" port:465];
[smtp setDelegate: self];
[smtp setMessage: message];
[message release];
ssl = YES;
mechanism = @"PLAIN";
[smtp connectInBackgroundAndNotify];