As far as I can tell, spamassassin-3.4.1_9 does not list all required p5
packages, which results in 3 error messages in /var/log/maillog upon
starting the package:


Feb 25 11:16:08 <mail.err> HOST spamd[69929]: Can't locate URI/_idna.pm in @INC (you may need to install the URI::_idna module) (@INC contains: /usr/local/lib/perl5/site_perl /usr/local/lib/perl5/site_perl/mach/5.24 /usr/local/lib/perl5/5.24/mach /usr/local/lib/perl5/5.24) at /usr/local/lib/perl5/site_perl/IO/Socket/SSL/PublicSuffix.pm line 124.
Feb 25 11:16:08 <mail.err> HOST spamd[69929]: Can't locate Net/IDN/Encode.pm in  @INC (you may need to install the Net::IDN::Encode module) (@INC contains: /usr/local/lib/perl5/site_perl /usr/local/lib/perl5/site_perl/mach/5.24 /usr/local/lib/perl5/5.24/mach /usr/local/lib/perl5/5.24) at /usr/local/lib/perl5/site_perl/IO/Socket/SSL/PublicSuffix.pm line 130. 
Feb 25 11:16:08 <mail.err> HOST spamd[69929]: Can't locate Net/LibIDN.pm in @INC (you may need to install the Net::LibIDN module) (@INC contains: /usr/local/lib/perl5/site_perl /usr/local/lib/perl5/site_perl/mach/5.24 /usr/local/lib/perl5/5.24/mach /usr/local/lib/perl5/5.24) at /usr/local/lib/perl5/site_perl/IO/Socket/SSL/PublicSuffix.pm line 134.


With that, spamd starts and seemingly works (I am not sure if some functionality is degraded).

Installing the three packages by hand got rid of the error messages.
p5-Net-IDN-Encode
p5-URI
p5-Net-LibIDN

The list of required packages at
https://www.freebsd.org/cgi/ports.cgi?query=spamassassin&stype=all&sektion=a
ll
does not include these three:

spamassassin-3.4.1_9
    Highly efficient mail filter for identifying spam
    Long description : Changes
    Maintained by: ports@FreeBSD.org
    Also listed in: perl5
    Requires: ca_root_nss-3.29.1, curl-7.53.1, gettext-runtime-0.19.8.1_1,
gnupg1-1.4.21_3, indexinfo-0.2.6, libnghttp2-1.19.0, p5-Digest-HMAC-1.03_1,
p5-Encode-Detect-1.01_1, p5-HTML-Parser-3.72, p5-HTML-Tagset-3.20_1,
p5-HTTP-Date-6.02_1, p5-IO-Socket-IP-0.38, p5-IO-Socket-SSL-2.044,
p5-Mozilla-CA-20160104, p5-Net-DNS-1.08,1, p5-Net-SSLeay-1.80,
p5-NetAddr-IP-4.078, p5-Socket-2.024, perl5-5.24.1, re2c-0.14.3,
readline-6.3.8

I am not sure if these 3 packages are needed for the port to function, but either they need to be added as required or the package needs to be patched to avoid the errors.