On 22/10/2020 11:33, R D S wrote:
Can somebody tell me...

Our web host had us using (for example) mail.bobsserver01.co.uk for
email server settings whereas Thunderbird et al autodetects
mail.ourdomainname.co.uk
Both seem to work equally well.

But when they move us to a different server (no idea, but it's at least
annually) I need to go around several PCs changing several email
addresses to mail.bobsserver03 etc.

I asked in the past and it was suggested their way was better with no
explanation but i've asked again today (we've moved again, shrug) and
they've conceded that it's fine to use mail.ourdomainname.

What, if anything, is the difference?

If you collect email via an unencrypted connection (say IMAP on port 147
or POP3 on 110), then both are fine - it sounds like your DNS records
contain a CNAME record that makes imap.ourdomain.com or
pop.ourdomain.com to the ISPs mail server(s)

If you use a TLS encrypted connection (usually ports 993 (IMAP), or 995
(POP3)), then using the CNAME version may get you a certificate warning
when you connect since the domain name contained in the certificate will
be that of their server and so not match your domain name.

--
Cheers,

John.

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