All the Write Email screens look like the following:

(1) At the top there are a set of buttons:
Send - sends the email
Attach - allows you to attach a relevant document (a select window will open allowing you to  find the required document from those belonging to the current customer/supplier/etc)
Attach File - allows you attach any file (a browse window will open allowing you to select any file on your computer)
Cancel - cancels the write email operation

(2) the From/To/CC/BCC/Subject block:
From - will display the preferred email address for the current Practice Location, but if you have multiple email addresses defined, then you can use the pull-down to select another. For example:

Note that in the pull-down, the name of the Practice Location is shown in brackets - but this is removed when the address is selected.
You cannot enter an arbitary From email address - you must use one of the practice location's email addresses selected from the pull-down.

To/CC/BCC - you can either use the pull-downs to select an address, or enter addresses directly.
If you use the pull-downs then the selected address in the pull-down will replace any existing address(es).  All relevant addresses will be shown. For example:

In the above case, the email was invoked from the Patient screen and hence includes the referring vet addresses. If this was invoked from the Customer screen, then the vets would not be shown. So use the pull-downs for quick and easy selection of the relevant addresses.
Note that in the pull-down, the name of the entity is shown in brackets so that you can see where the address is coming from. Hence the 2nd line shows Bill Bourke, but we know that this must be an email address listed for the customer Bourke,Johanna.  Again, the name in brackets is not included when the line is selected from the pull-down.  The last two lines appear as shown because the veteast.com.au email contact has the email name 'Email Address', whereas the veteast[at]gmail[dot]com contact has the email name 'Veteast'.

For more flexibility, you can also either enter an email address (eg bloggs[at]gmail[dot]com) or enter all or part of a name (eg bloggs,fred or bloggs, or blo, or %,fred) and then press the Enter key.  If there is a single match this will be filled in. Otherwise the Address Book screen will be displayed showing the matches.  Note that as indicated by the last example, you can use % as a wildcard; %,fred will find all who have Fred (or Frederick) as a first name, whereas %fred would also find names (like Alfred,John) containing 'fred'. If you press the Enter key with the cursor positioned on an empty line, the Address Book will open showing all email addresses in the system.
You can use multiple addressees separated by semi-colons (;).
You must have at least one To addressee, the CC and BCC addressees are optional.

Subject - enter an appropriate subject. Note that this is a mandatory field.

(3) the Text area - enter the text of the email. Remember that you can use macros here.

(4) the attachments area - as you add documents and files, they will be shown here. Note that if you want to delete one on them, click on the attachment to select it and then press the Delete key. Actually it's a little more tricky than this.  When the item is first attached it looks 'white' like the following: . If you click it in the middle (or anywhere except the icon at the left end), then the attachment becomes selected and the background goes cream like: . You can now press delete to delete it.

If you click on the icon at the left end, then the item will be downloaded to your browser (and displayed if possible).

Address Book

Below is an example of the Address Book screen.  The user has entered just 'sm' as the email address - the system checked and found more than one matching name and displayed them

Click on the required line to select the addressee. Alternatively enter a new search string and press Find.

If the Type is left at the default 'All', the address book shows all types.  You can use the pull-down to select just say Users (if you were looking for a staff member).

Note that the search is done on the name of the contact, not the email address or the email address name.

If the contact has an email address name, then this will be shown in brackets.  In the above, all the matching contacts have an email address name except the Supplier(Person) on the second line.  We can also see that Mrs Kay Smith is in fact a second email contact for the customer Smith,Michael. Note that the user SM has been included because his user name is SM, although he is Dr Sam Michaels.