Outlook shows both the fax numbers (where present) as well as the email
address when addressing an email. I understand that this was so you could
send a message via either transport depending upon your contact. Therefore
if you have multiple fax numbers or emails then the contact will appear
multiple times when addressing emails.

However, if you do not use fax on your system, then having the fax address
displayed becomes a nuisance. Both because it displays an entry in the
addressing dialogue window for each fax number configured as well as the
email address, and when you expect Outlook to resolve the name
automatically it does not know which of the addresses for that contact you
wish to use - showing the squiggly red line.

It is possible to suppress the fax entry in the address dialogue by
prefixing the Fax Number for the contact with a non-numerical character or
string e.g. Fax: 01234 345 678.

If you have a number of entries to manage in this way, then you may wish to
consider an add-in that will automate the task and allow you to select such
as that found at
http://www.websetters.co.uk/WSAddIns/Fax%20Display%20Manager/Index.htm

It resolves these issues by managing your contacts, hiding the fax entry
when addressing emails but still showing the details in the contacts folder.

Regards
Graeme
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