As long as your DNS records are pointing to the correct IP address, it shouldn't really matter for people connecting to your server from the outside. The important thing is that your website or email server can be found by those who you might want to connect to your server. Keeping unwanted visitors out is another story......
For email, there is a nuisance factor introduced by malware on infected computers, which commonly connect via dynamic (i.e. not static) IP addresses, and some email providers prefer to play it safe.
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