When comparing Sendmail vs Exim, the Slant community recommends Exim for most people. In the question“What are the best Linux mail transfer agents (MTAs)?” Exim is ranked 1st while Sendmail is ranked 3rd. The most important reason people chose Exim is:
Most Linux distributions that come with Exim, come with sane Exim default configurations. Changing default values and slightly adjusting configuration is relatively easy. On the other hand, building a completely new Exim configuration file from scratch might not.
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Pros
Pro Ability to use the "+" or "-" symbol in the local part of the email address.
Pro Ubiquitous
Due to inertia and historical reasons Sendmail is nearly Ubiquitous on all Unix platforms.
Pro Portability
Sendmail is the only major MTA that's fully ported to Windows.
Pro Easy to use
Most Linux distributions that come with Exim, come with sane Exim default configurations. Changing default values and slightly adjusting configuration is relatively easy. On the other hand, building a completely new Exim configuration file from scratch might not.
Pro Good documentation
Exim provides thorough documentation with great examples.
Pro Sendmail compatible
Exim is a drop-in replacement for Sendmail.
Cons
Con Missing some features
Missing featues like: webmail, ActiveSync, WebAdmin.
Con No support for some protocols
No support for: POP3, IMAP, POP3s (POP3 over SSL).